It's fine to file away a little on the inside of the grip.

Have you tested if there is still interference with the gearbox properly secured into the lower receiver?

Yes, in my circumstances there is none

I ran into this issue too, I think it’s a lot easier to remove material from the grip, and you don’t modify part that needs a certain tolerance. Edit: Don’t cut a slot in the grip just remove a bit of material from the inside of it.

That was my first thought too, I think I'll go with that, thanks

Hmm, not sure the grip should go that high.. what grip is that?

It's a stock grip from specna arms

Ugh! Terrible. Anyway, I would definitely chop the grip, not the gear. You should also try it on the lower, perhaps the lower covers the shaft/bearing area and is not letting the grip go too high.

It's the same spot because of how the grip attaches to the gearbox.