Gamecube uses standard DTV timings (email@example.comHz NTSC or firstname.lastname@example.orgHz PAL) in most cases (excluding 240p/288p games, some homebrew like GBI etc.) as far as I know. Those are also the numbers OSSC uses as its default sampling rates for 480i/p or 576i/p modes. The active resolution is more like to vary, but I’d guess 640×480 is most commonly used by GC games. If no PAL optimizations are applied, that results to incorrect letterboxed aspect ratio to begin with, just like with older 50Hz consoles. If PAL-optimized software output 768×576 active area, you’d need to increase H.active from 720 to 768 on OSSC’s 576i preset mode. That assumes your display can handle that resolution correctly.