I go thru the usual process of apply it thru this then I click on the shells in the scene tab to add the relevant minora and majora textures. Something isn't working though. I have it at High Resolution and Level 2 Subd for the viewport. To see if it helped.
View attachment 4403382
GP works much better with G9 then G8. GP and NG use similar "texture system" but I always prefered manual setting.
Seeing your renders I guess that you have some experience and you are able to set it manually on your own.
I do not remember how GP divide surface, but you should use 2 transparency masks with 2 UV. Can be done or separate materials (if exist) or on separate geoshells (if minora and majora use 1 material)
https://f95zone.to/attachments/2264115/
Samples above use 2 masks - darker crotch area (barely visible) and red/pink minora. Both maps have 40-60% transparency.
I see that GP have no majora/crotch transmap but roughness map can be used.