@pebcak I’m not skilled enough to see where it goes wrong, but the effort is still appreciated! 
@Svartis You were totally right in saying this, my bad. I’ll go ahead and correct my post.
So, I took a look inside grub.cfg and sure enough, the nvidia_drm.modeset=1 parameter was only being set for the two EndeavourOS entries (EndeavourOS and EndeavourOS fallback initramfs), but not the third Gentoo entry. So… I simply added the parameter there myself.
I’m not sure if this is something I should be doing but hey, it works. I’m convinced it’ll be overwritten next time I regenerate my grub.cfg, so I guess I’ll have to remember to re-add it manually.
I’m not sure if I just messed up my GRUB by doing this, or in case someone knows a better method, so I won’t mark this as the solution just yet. Cheers!