Congrats on the snag! There's a couple things to consider with what you're wanting to do. first, installing a ULE body might prevent you from keeping the site rail, because the outer dimension on it may be a hair different than the stainless body. You can get around that with a barrel adapter. You can get them from docsmachine.com, and they work very well, and are invisible from the outside. Then, to run a gas-thru grip, you will need a standard foregrip of your choice, and a vertical adapter. There's a few places to get those, but the best ones are either plain black, or black and say lapco on the side. I had one 32 degrees one, and it leaked. That may not be typical, though, so don't write those off based on what i say. When you're looking for the vertical adapter, make sure it comes with the screw, and that it only uses one screw. The R/T rails use 2 on theirs, and wouldn't mix with what you have.
I may be wrong on the body, so I'm open for correction, but that's what you're gonna need for the gas-thru. Good luck!