+1 for the M-Tac. I have one that will take all three Glock GAPs. Like Beanie, I prefer the 39 but the 37 carries in it just as well. I also like the C-Tac because it's a little thinner than the M-Tac. The other IWB holster I use sometimes is a Milt Sparks VMII for the 37 that I can carry all three in as well. Sparks takes forever to get in though. For the money, I would highly reccomend and Comp-Tac M-Tac or C-Tac.
My OD 39 is in an M-Tac in this picture. I have since switched to GAP floor plates.
All my Glock GAPs (37x2, 38x2, 39x3) are all Gen 3, I may have to get some Gen 4 as well...
Good luck in your search!