What are you trying to treat??
Chile - I am not aware of specific toxicity of gentamicin in fish. <ie not the usual renal & oto toxicity that you see in all humans when overdosed.> I would be interested in reading the source .... might learn a thing or 2.
I have not use gent in fish. Normally, I use antibiotics in food. I have dosage & recipes for goldfish, but not for livebearers.
Kanamycin is considered the best one for fish, and the dosage is 26-66 mg/l.
Aquavet - KANAMYCIN
In vitro tests would suggest that gent is ~3x more active than kanamycin:
Comparative Activity of Sisomicin, Gentamicin, Kanamycin, and Tobramycin
So in the absence of other evidence, I would try 1/3 the kanamycin dose.
Note that this is a conjecture. This is suitable as a starting dose for experimentation. I am not sure you want to experiment with your own fish!!
Note also that gent has too narrow a spectrum to be used without cultures. Personally, I would use Clindamycin instead (if I were to use an aminoglycoside). <Or just go buy Kanamycin - at least there is support for its use in Vet literature!>
EDIT:
Found this at the University of Florida:
http://edis.ifas.ufl.edu/fa084
The article specifically state that gentamicin is NOT effective in treating fish as a bath. They suggest you go use kanamycin instead! (preferably in food).