I am doing HPSP so I've been trying to piece together all of the military obligations and what they look like, so I just wanted to speak to doing a GMO tour as a con for USUHS.
I too thought doing a GMO tour was something negative cause I didn't really know what it was and things on the military websites made it seem like it was only for people that didn't match. It's actually a pretty cool opportunity, and a handful of people in HPSP/USUHS rank a transitional year as their top choice so that they can do one or more GMO years before going to residency. In some cases it lets you do cool stuff in operational medicine like fly airplanes or scuba dive if that's something that appeals to you. You can also end up somewhere pretty pedestrian like a stateside hospital if that's the kind of billet you ask for. In all cases, doing a GMO year makes you a lot more competitive for residencies since the residency program knows that you already have a lot more training under your belt.
IMO its actually more of a pro, but that also depends on what your goals are