GAPI should be built in to the x51v, but it's there for legacy purposes only. GAPI was discontinued after WM2003, it's only there now so that "old" apps that used to use GAPI still work. Now publishers are encouraged to use DirectX mobile instead.
It actually stands for "Game API" and it was introduced originally by Microsoft to make a way to program games for PDAs faster and more efficiently. It works for anything 2D/3D though including GPS software. If you've only got the choice of GAPI or no GAPI, GAPI is faster in most programs usually. In mapopolis I guess it would make the screen update faster/smoother.
It might be disabled because the program is detecting you have WM5. Or you might need to copy the GAPI dll file to the program folder. If you download Q3CE for x50 from this page
it comes with a file called gx.dll, that is the GAPI dll. Copy that file to the same place as the mapopolis exe on your axim.