It's approximately the same size as the Ghost, so it could really go either way. I'm going to hope it's a large ship though, as Huge ships are expensive and can't be used in tournaments. Maybe it would have ramming abilities?
The Gozanti didn't come out until well after the GR-75, in fact there was roughly a entire year where the Imperials didn't have huge ships at all so it sort of made sense for the first huge ship made to not have weapons and be a pure support craft.
On that note the GR-75 is far from doing nothing, with the huge ship only crew and upgrades it can dole out free actions to freindly ships, stress out swarms, deal unavoidable damage to stressed ships, and failing all that you can instantly crush unlucky smaller ships with little reprecussion. Sure compared to something like the CR-90 and the Raider it might not look impressive but it can still carry it's own weight and then some.
There are probably a few reasons keeping the Hammerhead back but they could be overcome in time.
1. Size, all the ships are made to the same scale so they don't look awkwardly sized place next to each other. I don't think this is a super concerning thing though considering that canonly a GR-75 is 90 meters long while a VCX-100 is about only 43, if you hold the models up next to each other a VCX-100 is more than half as long as the GR-75 so maybe huge ships have their own seperate scale (I don't own a CR-90 or any of the other imperial ships so I don't know where they look relative to the smaller ships). I also can't find a canon measurement of the Hammerhead but I'm fairly sure it's pretty long.
2. Playability, again this doesn't strike me as a huge hurdle to beat, I've never seen a tournament include huge ships and I've only played a handful of games with them myself but I see plenty of people using cards only found in their boxes. Still, FFG probably doesn't want to saturate their market with too many huge ships, this is a game about dogfighting, not two corvettes broadsiding each other. Though I doubt they will do it before another rebel and imperial huge ship are released the Scum did jus get their first huge ship so they're still 1 behind it would be nice if they focued on putting another one out for them so everyone is equal but Scum have always been a step or two behind.
3. Competition, the hammerheads were just released into X-wing's sister game Armada which does focus on big ship combat. FFG knows you want to see all those sweet big ships on the field but they would probably rather convince you to buy stuff for two games instead of one because $$$. This is probably the biggest hurdle as the company has to be convinced it's worth putting into both games.
4. Uniquiness. The hammerhead is a attack corvette, the CR-90 is an attack corvette. They gotta do something different, every ship in X-wing has a gimmick, doesn't matter if it changes the meta or it's never been more than a circus trick to pull for fun. Between the support capabilities of the GR-75 and the offensive role of the CR-90 they'd have to come up with something only the Hammerhead can do (a shame huge ships can already plow through other ships or that might've been a solid go-to). Probably the second biggest problem keeping the hammerhead out of x-wing.