Aren't GCM files simply renamed .ISO files?
Try going for ISO format files first, then go for GCM.
We still don't know for sure if other formats are gonna be read with the new modified Nintendont for Wii VC, so we have to take it step by step.
First ISO, then GCM if ISO works, and then FST or the trimmed format if both ISO and GCM worked.
Don't try to go directly for trimmed or converted ISO files because we won't know for sure if the problem comes purely from the badly converted ISO or something else in the injection process.