Hi and welcome to the forum
Yes the rainbowfish have gill tumours. There is no cure and I used to euthanise the fish when the tumours got too big or the fish had trouble eating, breathing, or swimming.
You can reduce the risk of them developing tumours by feeding them lots of plant matter and plant based foods. At least half their diet should be plant based.
Gill tumours can also be linked to Fish Tuberculosis (Mycobacterium species). There is no cure for Fish TB and a diet high in plant metter can reduce the symptoms that might appear.