Signal should be 625i (PAL) or 525i (NTSC). Something is up with the sync signal the OSSC receives. Gamecube has pretty standard output, so should not require any tinkering on OSSC. Verify you have a functioning cable and output from your Gamecube.
I contacted RGC and they advised that i had an earlier hardware revision & they have shipped me the newest revision. I’ll post back here if that sorts it in case someone else in the future stumbles on this thread googling for the same problem.
I don’t know anything about OSSC, but do know that PAL Gamecube does not output 480p (despite other regions supporting it, it was disabled in PAL consoles for some reason) , only 480i and 570i, unless you force 480p with Swiss or line-double / deinterlace with a Carby cable, Retrotink, or (I thought) OSSC… Don’t know if that helps.