When opening the Identity GUI in creative it fails to load Guardians Galore mobs and ctds it is also possible that his mobs were intended to not be played perhaps

Most likely a client-side crash from Guardians Galore... I need to get around to patching hard crashes like this anyways, so I will try to prevent the error on my end. If you provide a crash log, we might be able to fix it on their end as well.

Yes obviously, this only works with vanilla mobs

Are you able to reproduce this issue? If so, can you please post a full crash report or log from reproducing the issue?

Fixed on most recent version of Guardians Galore.