so , the only benefit to run retropie on 64bit system (on raspberry) is use dolphin-emu
Check out this thread if you really want to push for Gamecube on the Pi.
From reading that, some games work ok, but many others are not likely to run well enough to enjoy playing them. Running Dolphin is a pretty big job for a Pi 4.
EDIT: As development continues on the various drivers and even Raspberry Pi OS 64-bit, this situation could improve. But keep in mind many of these things are outside of the control of the RetroPie project.