Gentamicin is an aminoglycoside antibiotic that has been around for a long time and has a few problems associated with it. First, due to it's lengthy existence, many species of bacteria have developed a resistance to it. Secondly, of all the aminoglycosides, it probably has one of the smallest spectrums of activity against pathogenic bacteria. And lastly, all aminoglycoside antibiotics are quite toxic, but gentamicin is slightly more toxic than the others in the group. If you must use an aminoglycoside, amikacin would be a better choice. It has a much better range of bacteria susceptible to it than gentamicin, and is slightly less toxic. Tobramycin is also another aminoglycoside that is a possibility and is sometimes used as an alternative to amikacin. Bloods are more sensitive to aminoglycosides, so dosages should be half for them (1.25 mg/kg) than that used for other python species, and no more frequently than once every 72 hours. Also, always have fresh water readily available to your snakes when using aminoglycoside antibiotics.
Due to the above toxicity concerns, unless there is a good reason for using an aminoglycoside, there are other cephalosporin and quinolone antibiotics that would be safer to use. Make sure you consult a qualified reptile vet before starting any drug treatment. Good luck.
Kelly