The GX-5's are more of a keep it all stock, vintage classic that you wouldn't want to modify at all as they fetch top dollar in original condition. While definitely almost the sweetest 1200, the only thing they're good for these days in my book is car shows.
The GX-5 1200 is only different than a standard 1200 in a couple ways. You get a cooler trim package, the GX hood, mirrors, dash, seats, and steering wheel, a 5 speed transmission, a quicker advance curve, a little bigger cam, SU's, the GX head, and a better flowing exhaust manifold. It's quicker and better looking to start off with but you can perform all these plus more mods to a standard 1200, get better results, and not have people hunting you down for molesting a GX.
The better and more rare 1200 to look for IMHO would be a Sunny Excellent GX! They're a little longer in the front end and the grill and lights are placed a little differently, but overall they look the same and they came stock with the L14 engine (hence the longer front end) dual SU's, H165 rearend, and a 5 speed! I wish they were sold abroad like the standard 1200, all that extra room in the engine bay would make swaps a LOT easier.
I have only seen 2 Sunny Excellent coupes in two years in Japan, and only one was a GX, but they sold for less than the GX-5's go for. I've seen at least 5 or 6 GX-5's since I've been here, definitely more of them around than the Excellents.