Well my name is Christina and my younger sister is Sabrina and they sound so similar tbat our wholes lives there was always some one who switch our names and it was such a annoyance for us. Give the boys names where people wont feel confused about who is who lol if you are adamant about a G name make it Completely different!
I agree with AnnieBrooke, so i would avoid Gus (even tho it reminds me of GusGus the mouse in Disney's Cinderella, and he's cute) Greyson and Grant are okay tho, not to much alike Guy. Out of those two i prefer Greyson. But think outside of the G-box too, many nice names out there!
Your second child will be an individual, a different human being than the first. Give him a chance at some identity of his own with his name. Don't make them a stunt set of names like my neighbors Tim and Tom, the poor guys.