I can't speak for the USMC as a whole, but I know the GMO's at my MAG have a pretty sweet gig. We treat all docs the same (Flight Surgeons and GMO's), meaning we all get 50/50 time (2.5 days of clinic and 2.5 days of "squadron" per week). By nature, GMO's have relatively fewer collateral duties and responsibilities around the squadron, so the GMO's tend to have pretty cush jobs (as do us FS). Typically, flying squadrons will have a designated FS, and the non-flying squadrons may use GMO's due to the FS shortage. For example, MWSS/MACS/LAAD/HQ/etc. As for FitRep, your CO will sign your FitRep as a 1 of 1, so you won't be ranked against anyone. That can be a double edged sword, but at least you won't get "the worst FitRep in exchange for doing the most work." Getting your FMF looks great on uniform and on application/promotion packages.
Feel free to PM me if you have more questions. If you know what location you're looking at, it's pretty easy to get in touch with the docs there to get the best inside gouge.