How to extract wavefunction from GAMESS output?
I'm not too familiar with the input structure of GAMESS. But, I found and input in which you can find how to call the AIMPAC keyword GAMESS input with AIMPAC keyword and this is how wfn file looks like. In principle, when you indicate the AIMPAC keyword, the GAMESS itself print the wave function in a file with wfn extension at the same folder of the input/output.

After you get the wfn file, you have to integrate it with (I recommend) 1. AIMPAC or 2. MULTIWFN (Both are free).