A lot of my attendings at the military hospitals are GS. They definitely make more than the active duty but less than their private civillian counterparts. Usually they are retired military that appreciate the setting/community and don't feel like having to deal with insurance and such. Other things they mention include better PTO (30 days/year), better hours, more collegial atmosphere, and continuity of care.
They make less than civillians because government. Tax payer funded, budgets, and the fun goes on. Also, rotating at civillian vs non-civillian hospitals the physicians are pretty similar but the efficiency of the ancillary staff, support staff, admin, etc. is night and day. When I was on GI the civillian location and militiary hospital were staffed by the same attendings but the civillian location could do a number of scopes that would take the military hospital all day in about 2-3 hours.
Note I'm not talking about the VA, that's a whole other ball game. Some of the above apply but.....well..... you all have seen the news.