Tetra is the one that was recommended to me. Fish Tetra should work as well, it's still the same antibiotic after all. If you can get oxytetracycline, that's even more effective apparently
You could certainly try tylan but it's no where near as effective. It'd still be better than not treating, in my non- vet opinion.
If you can get ethromycin, that's also recommended as a treatment (I know API make one for fish - might be called P-Furan??? I can't remember as it's no longer available in the U.K.)
Stubborn infection can be treated with Tetra and penicillin in combination according to a classmate who worked in the poultry sector in India, where GD is rife. Worth considering?
Where antibiotics aren't working so well, use organic acids to acidify drinking water. ACV would work. Not sure you should add it with the antibiotics though.
I hope something works for the little sweetheart, keeping everything crossed