I love FreeCiv. It's cost me more of a GPA than anything else, and that includes nethack (which I would also love to see a port of, maybe that's what I'll do).
It's definitely possible performance-wise, and the GP32 could definitely handle it, though from my experience with the app it's pretty heavy on reliance with different windowing toolkits (GTK+ and XLib versions are available).
Maybe the windowing stuff is all object-oriented and can easily be swapped; I haven't taken a whack at the source code.
A Freeciv clone would be easily within the realm of possibility, and perhaps even a port if someone is bored enough to write replacements for all the UI code.