Let's say I manage to expand (opposite of compress in this context) the source code of a GPL-derivative I made to an unusually large amount that is nearly impossible to download (let's say a googolplex EBs). Storage is not a problem for me as I'm generating the expanded file on the fly. Additionally, I'm limiting the download speed to 1 bit per year. Can I be sued for violating the GPL?
The GPL does not say 'pretend to make source code available'. The means by which the source code is made available must be equivalent to the means by which the compiled program is made available.
Relevant clauses include (in version 3 of the GPL) clause 6, which says:
You may [distribute your program in object form] provided that you also convey the machine-readable Corresponding Source under the terms of this License...
The conveyance of source code must be in one of the prescribed ways. The one most relevant to the question (and which is illustrative of the issue at hand) is that in clause 6(d):
[You may convey] the object code by offering access from a designated place (gratis or for a charge), and offer equivalent access to the Corresponding Source in the same way through the same place at no further charge. You need not require recipients to copy the Corresponding Source along with the object code. If the place to copy the object code is a network server, the Corresponding Source may be on a different server (operated by you or a third party) that supports equivalent copying facilities, provided you maintain clear directions next to the object code saying where to find the Corresponding Source. Regardless of what server hosts the Corresponding Source, you remain obligated to ensure that it is available for as long as needed to satisfy these requirements.
The phrase 'equivalent access' means that, if you are only prepared to provide source code in an arbitrarily padded format over a very slow network link, then that is the only form in which you can provide the object code. So you will have no customers.
Where you convey your software via other means such as on disk (the other subclauses of clause 6) there are similar requirements for equivalency.
Your question should not be "can I be sued", but "will I be sued".
Very often, the copyright holders of GPL licensed software just want you to conform with their license. So if they see a breach of license, they will send you a nice letter, then you either read the license carefully or you ask a lawyer who will tell you that you have no chance to win if things go to court, and then you conform with the license. It's very rare that people get sued in these cases because that's not what the copyright holder wants.
In your case, if I was the copyright holder, I would be so pissed off that I would take you to court with the goal of getting every penny that you own. And if this goes to court and you tell your legal theory to the judge, you will make his day.