I like Gunnar because it is more unusual. That said, I don't think it is from the same ethnic background as Levi, (I think it is Scandanavian? While Levi is from Judaism?) and I don't particularly like the play on "gunner." The idea of a gunner makes me think of someone who was drafted in World War II to serve as a gunner on an airplane. In other words, a grunt.
I also like Grant, but it is less unusual than Gunnar, and there was an actress named Lee Grant so I often associate Grant with a girl's name even though it was her last name. Don't know why Ulysses Grant doesn't spring immediately to mind, but it just doesn't. lol
Do you like any other G names? My friend Garland would vote for Garland (a great Southern last name), my friend Gideon would point out the unusualness of his name and its tone's similarity to Levi. There are also Griffin, and Gage, and Gerard.