thanks for posting the pics as those do help, if you can get some shots from the side that would also help.
first a few basic pieces of info would help, especially since you already lost one gar to apparently the same thing:
- how big is the tank? what is the filtration on the tank, and how often (and how much) do you change the water?
-what are the current water parameters? pH? ammo? nitrite?
-what are the tankmates? i can see the RTC, and that is already not a great choice as it would easily grab the gar, regardless of whether it could eat it.
-how long has the gar had this condition, and how long did the older gar have it before it died?
this looks like an infection which stemmed from some physical damage to the snout, so something likely got into a fight with the gator (or the gator may have picked the fight) and the resulting injury got infected...which also suggests there may be some water quality issues.
the initial treatment i would suggest is do a 50% water change and then add some salt to the tank...about 1 tbsp/20g of water. increase the temperature to 82F and we can go from there.
keep us posted, and good luck...gars are tough, so hopefully we'll be able to help it bounce back--
--solomon