(update: we found out what it was! the extension wasn't handling different languages properly, this is fixed in 4.2.0. 🐺)

when you go to the mod page, GXM should take over the button that normally asks you to install Opera GX. here's a page as an example: https://store.gx.me/mods/iid45j/gx-boy/
it should have a red button saying "Install with GXM"! if it doesn't then something's gone wrong, and i'd really appreciate if you could open a bug report so i can help you further: https://github.com/noblereign/GX-Mods-Extension/issues/new/choose