The gg TLD zone itself is signed and there do indeed seem to exist signed child zones as well. All of which indicates that it actually is supported.
The question is not entirely clear about what exactly it is that "isn't supported" by the registrar and/or DNS hosting provider that you are dealing with. So I'm breaking it down into the two cases that I can imagine:
Delegation
If this is about the delegation itself (effectively the ability to add a DS record alongside the delegation NS records in the gg zone), it would seem to indicate a limitation of the specific registrar you are using (even if they themselves possibly phrase it differently).
See for instance this listing here as a counterexample to the "it's not supported for gg" claim.
(I'm sure there must be many other options for registrars that support this, I have no affiliation or interest in your choice of registrar, but wanted to show a concrete counterexample.)
If this is about what is clickable on the registrar site, it could also be that your current registrar just does not have automation in place and can do this manually for you, but if you have been in contact with them and they say it is not supported, I guess that is not so.
DNS signing
If the limitation instead is about the actual DNSSEC signing operation, it's not really about the gg TLD itself, rather about the DNS hosting provider (possibly the same company that is your registrar, but not necessarily so) not providing DNSSEC signing as part of their DNS hosting service.
If so, you could use a different DNS hosting provider also without changing registrars.
