I'm not sure if GC batteries are the only true deep cycle batteries out there. Maybe manufacturers do just print different labels. But, I guess I'd want to be sure we are distinguishing true deep cycle batteries from dual-use batteries.
Regardless, here are the reasons I like golf cart batteries for the house batteries:
1) Cheaper per 12V amp-hour than other deep cycle batteries.
2) Easier to carry onto and off of the boat than larger batteries.
3) If a cell goes bad, I have to replace only a smaller cheaper GC2 battery, rather than a larger, more expensive battery (that is basically like the GC2 packed in one).
4) Sometimes more options to squeeze smaller boxes into nearby spaces than comparable larger battery box(es).
Down side(s) to GC2 batteries as compared to /large/ 12V batteries, e.g. 8Ds:
1) Even though 2 GC2 batteries fit in a box, larger batteries tend to waste less space per box, because of the "extra" space around each battery in the boxes. So, I think larger battery boxes have more energy per box volume.
2) Tying 6V batteries together takes more wiring. This is true both for the "jumpers" to combine the 2x6V batteries in series into one 12V battery, and then the wires needed to tie multiple of those together in parallel. This is more time spent cutting and crimping. And more posts to keep clean of corrosion. And, more crimped connectors that can corrode or weaken from mechanical stress. And more clamp connectors that can loosen.
3) There are more boxes to open and close to check water as compared to larger 12V batteries.
All-in-all, I am a fan of the GC2 approach. Mostly because of the price per A-Hr. But, my current boat has 4D dual-use batteries. When they go, I'll probably replace them with deep-cycle house batteries if I can find them. I don't know where I'd put a bunch of GC2 in boxes.
Of course, if you aren't going to box the batteries, that does change things a lot.