Inquisitor Seal is one of those skills you almost always max out, especially if your other class is on the squishier side. It’s pretty much 3/4 of the defensive power that Inquisitor brings to the table, and makes you almost immune to DoT.
There are a lot of better infiltrator builds (top 3 that come to mind are cold shadow strike, Belgo set, and that pierce RoK conduit), but folks have pushed cold gunner to SR30 and some celestials before and I see no reason you couldn’t get there yourself. As long as you have a pair of the actually good cold pistols (M. Hagarradian Enforcer) it should be able to do it, though you might be spending some time farming purple Alkamos rings because the blue ones don’t work well for a ranged build.
Hopefully FoA brings in some better cold guns, because there’s basically no alternatives. Arcanum Frigis is for casters and Shadowspine Enforcer is really bad even for a blue.
Specifically thinking of these posts, if you want examples of successful final setups -