Quite simply: what do you think? 
I like it. I don’t like the obvious nn [name_m]Greg[/name_m] though.
The Mountain? (Game of Thrones) Not necessarily a good connotation, if you’ve read the books. I’m not a fan of the nn [name_m]Greg[/name_m], either.
So that’s a no from me, I guess.
EDIT: Haha it is The Mountain (thanks for pointing that out!) and yes I am guilty of Theon so I guess I can’t talk. <3
…said she who has [name_m]Theon[/name_m] in her top names. 
I like [name_m]Gregor[/name_m]. It’s a solid, distinguished, masculine choice, and very rarely heard in the US. You could use [name_u]Grey[/name_u] instead of [name_m]Greg[/name_m] if you want a nickname, although I do prefer [name_m]Gregor[/name_m] in full.
I like the Scandi variants of common, dignified names. [name_m]Gregor[/name_m] fits right in with [name_m]Anton[/name_m], [name_m]Marius[/name_m], etc. Kind of cool & Continental, and fresher than [name_m]Gregory[/name_m].
(I also think of genetics via [name_m]Gregor[/name_m] [name_m]Mendel[/name_m]).
No I don’t like it. I much prefer [name_m]Gregory[/name_m] but not with the nn [name_m]Greg[/name_m]? I prefer the nn [name_m]Rye[/name_m] for [name_m]Gregory[/name_m] even though [name_m]Gregory[/name_m] ends in a long e sound. [name_m]Regis[/name_m] could also be a handsome and unique nn for [name_m]Gregory[/name_m].
The Hound’s name is [name_m]Sandor[/name_m]. [name_m]Gregor[/name_m] is his older brother, also know as “The Mountain.”
Thankyou everyone. I’m glad I’ve got honest feedback here. Some god and some bad. I do like [name_m]Gregor[/name_m]. I prefer it to [name_m]Gregory[/name_m], and I’ve never really known much about Game of thrones 
I don’t like [name_m]Greg[/name_m] either, but [name_m]Gregor[/name_m] in full seems masculine and strong. 
I love [name_m]Gregor[/name_m] far more than [name_m]Gregory[/name_m]. I do think of [name_m]Gregor[/name_m] from [name_m]Kafka[/name_m]'s short story Metamorphosis. He turns into a cockroach.
[name_m]Gregor[/name_m] is a great name and I prefer it to [name_m]Greg[/name_m] or [name_m]Gregory[/name_m]. I feel that it is strong and masculine and I love the look of the name.
I like it very much but think it would often be misheard as [name_m]Gregory[/name_m].
I like [name_m]Gregor[/name_m] but not [name_m]Greg[/name_m]. I like the suggestion of [name_u]Grey[/name_u] as a nn.