Gorilla glassglass 3 of 630 shld be enough to protect your phone from all sorts of scratches. But if u are a really rough user it wld still be a good idea to get that extra protection.
But the funny thing is that most nokia care guys actually advise against putting screen guards.
I bought my phone online and didn't come with a screen guard. I went to the nearest nokia care to get one.
The guys over there told me u don't need a screen guard as ur phone got gorilla glass. The fact is 620 doesn't have gorilla glass and it took me a while to convince them to actually put a screen guard on it. If that's the case with my non GG 620 then I bet it will be even harder to convince them to put screen guard on a GG3 630
If you put screen guard on the screen, the gg will be not in use(only contact with protection foil) so the foil will get scratched. It will be same as other non gg devices.