^Yeah, most vanguards don't opt to take the GPS into combat. It has a press and release shot that has a couple millisecond delay. Vanguards learn to hate that delay. It also does less damage on a uncharged shot than the eviscerator does(vanguards shouldn't be charging shots all the time).
Personally, I don't consider the GPS to be a shotgun. I consider it a baby heavy weapon that uses thermal clips and I have no idea why they made it. Christina Norman described it, along the lines, as a shotgun that players who didn't want to get up close could use. : / That's kinda what a shotgun is in gaming. You get up close and your rewarded with high damage. The GPS has the range of a tempest, but hits like a shotgun. That combination CAN(not will) discourage "real" vanguard play especially in new vanguard players.
Pretend a mob is 25 yards a way. A "real" vanguard would gtfo of cover, charge that f***er, blast him in the face, and give him an elbow for good measure. Now a vanguard with a GPS shotgun could just as easily aim out of cover and give that mob a little "pshwp..pshwp..pshwp."

But that's not what vanguards do and that's the main reason I don't favor it. Like I said, it does less damage than the eviscerator on an uncharged shot and in the hands of a vanguard it's only overpowered in the range department. This is where the overpowered arguments come in though. A thermal clip fed gun with that much range shouldn't have that much power and vice versa. On top of all that, the GPS tracks your friggin target if you aim close enough...that's soooooo not vanguard. >
Sorry for the rant. I've been holding that in. Use it if you want to though. I hold no qualms against GPS users. It's just between me and the GPS.
TL;DR Cuz the GPS ain't a shotgun yo.
P.S. In the end, "real" vanguards use the Claymore.