G&p ones are hit and miss, I've had two, both turn themselves off with the slightest knock (which is an issue on a tm recoil ) , two friends has the same issue with theirs too. I know Sam_G on here had problems with his as well, can't remember what exactly.
End of the day they're just rebranded Chinese optics, hence the variation in quality, same with WE and most other makes. Find a cheap one you like the look of and hope it's decent.