Grand Theft Auto: San Andreas was never officially released for the PlayStation Portable (PSP). Files labeled as "GTA San Andreas PPSSPP ISO" are typically fan-made mods—often modifying GTA: Liberty City Stories or GTA: Vice City Stories—or are disguised Android APK files. How to Install GTA San Andreas Mods for PPSSPP
If you already have a legal UMD copy, here’s how to create/obtain a properly working ISO for PPSSPP:
If you downloaded a "Highly Compressed" or "Fixed" version to avoid lag, you still might need to tweak the PPSSPP settings:
Obtain the ISO/Zip File: Find a highly compressed version from community sources like BOKA GMZ or PCQuest.