Definitely
Gregor:
George is just another name that starts with G. It's got nothing to do with your grandfather, and the only way to honour him is to use either his own given name or, if it works for you, his surname.
Ronan Gregor sounds good; much better than
Declan Gregor, with that repeated e sound. I'm glad you want two mns - my children have two each and it's worked well for them. Without knowing your surname, it's hard to come up with a combo that flows well, but here goes:
Ronan Gregor WilliamRonan James GregorRonan Gregor SebastianRonan David GregorRonan Gregor Hugh