I was also concerned about ease of fitment before I bought mine, however the concerns were unwarranted.
In a RFSV the Exmoor Trim seat sat on the two outer seat rails. It required no modification or movement of the existing seats, and doesn't even require removal of the floor part of the rifle holder. On my RFSV was extremely easy to fit.
It is bolted to the floor by 4 bolts one in each corner so will need 4 small holes drilled in the seat box, and a plate under the floor of the seat box.
As a warning, it is a child sized seat and is not very comfortable for adults or large kids because of the legroom.
I fitted a 3 point harness seat belt which was 90% of the fitting difficulty due to fitting a top seatbelt bracket.
If you are looking at buying make sure you look at going direct through Exmoor Trim as an option, they are easy to deal with and freight was reasonable and very quick.
I don't think a GS would require much more effort to fit, if any, but I cant confirm as I don't have one.
cheers David