Spelljamming ships indeed don't "founder" in space (a holed hull really doesn't do anything except make for expensive repairs), but gammaroids conceivably can be found planetside/in atmosphere, and could attack sailing ships. (Yeah, I know - very rare, and missing the point of a gammaroid, but it could happen...)
Yes, but a gammaroid couldn't attack sailing ships with its Ramming attack, since the description says both it and the target have to be airborne.
Also - I like 12d6, but I'm fine with anything 'around' that mark. That's an average of 42 before fire damage, which is a nice ding. Wood (most ships, spelljammer or otherwise) only has a hardness of 5... Admittedly, 12d6 isn't very impressive to high-level characters who would be fighting this thing. But ramming characters would be dumb for it to do anyways... it seemed pretty clear that the point of ramming was for ships. Characters get the bite/claw/claw routine.
So yeah... 12d6?
12d6 seems a bit weak.
Its bite/claw/claw of 16d6+32/3d8+16/3d8+16 would average 147 damage assuming all of its attacks hits - which seems likely considering it has a +125/+120 attack! - and the 84 point average damage of 12d6 + 12d6 fire is a little over half that, being about 57% of its full attack damage.
The ratio'd be even worse if the Gammaroid was Power Attacking or the target has fire resistance running.
However, I'd think the gammaroid would add a Strength modifier to the damage in a similar manner to a Trample attack, either Str×1½ or Str×2. The former would give it 12d6+48 plus 12d6 fire (average total 132), or about 90% its full attack. The latter would give it 12d6+64 plus 12d6 fire (average total 148), which is almost the same as its 147 point full attack average.
Speaking of trample, I fancy using bigger dice for the impact damage much like most trample attacks do.
Say, 6d12 instead of 12d6, for 6d12+48 or 6d12+64 damage for the impact plus 12d6 fire, which'd produce an average damage three points less (129 points or 145 points).
Alternatively, I'd also consider a lower ×1 or ×1½ Str bonus multiplier and a few more d12s, e.g. 8d12+48 plus 12d6 fire (average 142).
