I'm getting ready to assemble a DPMS rifle kit onto a Raptor Billet lower receiver.  When I put the upper and lower together to test the fit I noticed that there's a gap, a hair less than 1/8th of an inch, between the rear of the upper and the part where the receiver extension screws into the lower receiver.  This is with the take down pin holes aligned.  Something to be concerned about or a cosmetics issue?  Think there will be an issue with gasses blowing out through the gap?

Nothing to be worried about, not all recievers will mate perfectly.

Yes 1/8” is something to be concerned about.
This is way off. One or possibly both are out of spec.
IMO even if it is functional you do not want this.

Could be. If the carrier does not push the buffer off the retaining pin when the upper closes the buffer detent will contact the edge of the buffer every time you fire it. If it does push the buffer off the detent like it should then it is probably just a cosmetic thing.