Your grandpa does not sound like a nice man!
Seek advice - I believe some juristictions permit the owner to evict tenants so long as the owner wants it for their own primary residence. Wanting the home for his girlfriend would fail to meet those conditions.
You could help any legal advice by going to City Hall, get a copy of the title deed of his current home that he lives at. Bring that title deed and the civil suit paperwork that you received to your legal advisor. If my understanding is correct, then any claim on the home by your grandpa would be on weak legs since he already had a primary residence. A legal advisor could also get a copy of the title deed but would charge you for this effort, hence why I suggest you go prepared.
I do not believe your relationship with your grandpa helps or hinders your situation however if he bought your home, surely there must have been a common understanding on him doing this to help you out and that you would live in it for X years until you got on your feet again. If such a conversation took place you should bring any evidence with you. Even without evidence, tell the lawyer as while limp, verbal agreements have a place in many courts.
Getting you out should not be a quick affair (especially if you have kids living at the address). Just don't ignore the civil suit as otherwise he could win by default.
Best of luck!