Who Is At Fault?
The biggest problem here was your own lack of due diligence. You should have hired a lawyer to review the title work and survey (which probably would have cost $1,500 or less and would give you a very solid malpractice target if he or she missed this issue). The basic rule in purchases of "used" real estate is caveat emptor, i.e. buyer beware. Tuition in the school of hard knocks is expensive.
Unless you can prove someone else is legally at fault, you are at fault legally.
Not the Bank's Appraiser
The bank's appraiser very likely doesn't owe you a duty (his appraisal probably says it may only be relied upon by the bank), is entitled to rely on third parties re property features without independently reviewing a survey, isn't required to look actively for exceptional property features not made known to him when he was engaged by the bank, his contract almost surely had strict limits of liability in his contract with the bank, he probably looked at comparable properties in the same complex that controlled for this problem, and is also insulated from liability since he valuation is an opinion that can't be fraudulent by definition.
Not the Home Inspector
Normally, an inspector is looking for things that a broken, not easements of record, and normally a house inspector has very strict limits of liability by contract to the amount of their fee which is usually a few hundred bucks at most. This is outside his job description and the inspector is not an insurance policy against defects and undisclosed problems.
Possibly A Buyer's Agent, If Any
Your buyer's agent (if any) may have had some due diligence duties or at least a duty to advise you to have a lawyer review the title documents and plat (obviously, if you had no agent you can't sue that non-existent person). It isn't clear if there was a buyer's agent involved at all, however. If you had a broker but he or she was a transaction broker instead of a buyer's agent, however, he or she probably didn't have a duty to make sure that you did your due diligence. Also, if your buyer's agent or transaction broker told you to run the paperwork by a lawyer and you didn't despite that recommendation, the buyer's agent probably is off the hook and did his job.
Not the Title Company
The title work almost surely disclosed a plat showing the easement, the plat was almost certainly a matter of public record, and the title company almost surely excluded easements of record from your policy.
A physical inspection of the property would also give you a duty to inquire regarding the gate, even if it wasn't clear that it was an easement.
Seller or Seller's Agent
Together these two facts means that mere silence or non-disclosure from the seller or seller's agent isn't enough to prevail in a lawsuit against them, and means that even a knowing misrepresentation will not necessarily prove liability, unless their statements were intended to and did cause you to refrain from doing due diligence about the fence. Liability to them could also be tough if there were written disclosures that weren't affirmatively false even if they didn't mention this, and there was an integration clause stating that only disclosures made in writing may be relied upon.
But, the focus on the seller is because the seller is responsible for appears to have made the misleading statement (because the seller made it, or because the seller is responsible for statements made by his or her agent), and because most of the benefit of you not knowing benefitted the seller and to a much lesser extent, the seller's agent.
If the seller and the seller's agent were silent about the gate and you heard wrong about it from a third-party, you probably have no case against them, and also no case against the third-party upon whom you were not entitled to rely. You need an affirmative misstatement of fact to prevail against them. If someone showing the property on behalf of the seller's agent made a misstatement out of ignorance and never mentioned it to the seller or seller's agent, there is also probably an insufficient basis to sue them.
If the seller or the seller's agent knowing lied to you, hoping that their statement would cause you to ignore the plat showing the easement, you might have a case against them, but it would be a hard case because you had full access to public records with facts contrary to what you were told which are constructive knowledge to everyone. It isn't enough to show that they told you something that was wrong. You need to show an intent to prevent you from finding the truth and a knowledge of falsity on the part of the seller or the seller's agent (the seller's agent is also entitled to believe the seller without confirming it, so the seller's agent only has liability if the seller's agent knew that this was fault; the seller in contrast is responsible for the seller's agent's statement is either the seller or the seller's agent knew it was false and tried to deceive you).
It may also be possible to sue the seller, or seller's agent even if that is who you communicated with, for negligent misrepresentation (instead of fraud). Negligent misrepresentation is sometimes covered by insurance, while fraud is rarely covered by insurance, and insurance coverage is important if the defendant is not collectible otherwise. There are no punitive damages if you prevail on negligent misrepresentation claims but not fraud claims.
Evaluating the Case
Odds of winning
You have either barely a 50-50 case or worse depending on the exact facts. The seller can point fingers at contributory negligence by you, or non-parties at fault like your broker.
You also need to determine if there is a mediation or arbitration clause in the purchase and sale contract (if you sue the seller), or in the contract with your broker if that is who you sue (and if you do you definitely need an expert witness on a broker's standard of care).
You also have a statute of limitations ticking away, which may be counted from the time of the misrepresentation, rather than the date of the sale in a worst case scenario, so if you sue you need to act sooner rather than later.
Costs Of Litigation
This case is not one you have any meaningful chance of prevailing in if you bring suit without a lawyer and an expert appraiser at a minimum. Realistically, it will cost you $30,000-$50,000 of attorneys' fees to take a case like this to trial, and it is unlikely that you can find a lawyer to take the case on a contingency. You also need to hire an expert witness appraiser to prove your damages which will cost $5,000-$10,000 or so (I am skeptical that your damages are as high as you believe them to be, I'd be surprised if your damages were more than $40,000-$50,000). Some states require an expert witnesses certification in advance or early on if you sue a licensed professional which can be expensive (ca. $5,000-$20,000) and hard to find, and if so you may decide to sue the seller but not the seller's agent.
Length Of Litigation
It probably takes 9-18 months to get to trial, and longer is there is an appeal, but shorter and with less expense if there is a pretrial settlement.
Likelihood Of Pre-Lawsuit Settlement
A settlement in response to a mere demand letter, even from an attorney is unlikely unless the demand is very low and well substantiated, and if you make a low opening settlement offer you are unlikely to ever secure a bigger settlement than your initial demand later on and may have to compromise a little to get any deal at all no matter how low your initial demand may be.
Best Case Scenario Win
If you win you get the difference between the price you paid and the fair market value if it didn't have the concealed defect (probably $40,000 to $50,000 or less). You might get your attorney's fees and/or punitive damages equal to up to one or two times your actual economic loss. If you win, the loss probably can't be discharged in bankruptcy (but you will have to litigate the issue in bankruptcy court if the judgment debtor does go bankrupt on a very short and strict deadline). But, the seller may or may not be collectible (normally, someone selling a condo that is their residence for $200,000 isn't all that affluent, particularly if there was a decent sized mortgage paid off at closing).
Worst Case Scenario Loss
If you lose you probably end up paying their insurance company's attorney's fees and costs and possibly expert witness fees under the terms of the purchase and sale contract (probably similar to your fees and costs).