looks like damage to the back, probably from fast-start into the wall of the sump. gars are much more susceptible to this sort of damage when young, but seem to be able to recover better as well.
unfortunately YOY CBGs are very skittish...even moreso than most other gar species. even with those that feed or try to feed, this level of damage does not seem like something they recover from. i believe it can also be tied with malnutrition and also chemical issues in the water, but these more or less exacerbate or increase susceptibility to the broken back/damaged back, which is the primary issue.
with Cubans we have just had to put down fish that ended up like this unfortunately, as their skittish nature only tends to make the condition worse. if you would like to try and save the fish i would recommend keeping it in a shallow amount of water and see if it will eat. keeping it in the sump with no light probably made it a bit more skittish as well.
sorry for your luck, they are great but sensitive fish--
--solomon