Haven't done that particular flight yet but I have been to both Galway and the Generator. I looked for anything out of Galway and couldn't find anything so I'm guessing that bussing it to Dublin and going from there is your fastest and probably cheapest bet.
The Generator is in the northeast part of central London but it wouldn't really matter which airport you fly into. If you look at a map, Luton is due north of downtown and Gatwick is due south but they're both about the same distance. You might have to change trains fewer times flying into Luton but if the fare flying into Gatwick is cheaper I'd do that cuz I'm assuming that the travel time on the trains would be negligible.
Have fun, Galway is awesome. There's a bar called, I think, "The King's Head," or something like that that is pretty sweet if you're looking for a busy, live music atmosphere and a couple bars across the street that can be a bit more mellow. Also, if you have the opportunity to drive through Connemara to the north of Galway I would recommend it. Don't need too much time but the area is gorgeous.
Live life so there is standing room only at your funeral
The beauty of this world will last
Much longer than its men
No need for man to test this fact
Enjoy the view instead----Me(Part of a longer poem)