I know this answer might not be the most helpful, but from someone that raised groups of baby gars a few times, sometimes one just dies out of the blue. To me it actually happened again with my tropical gars some days ago.
I have no idea why, maybe they hit the glas, swallowed something to big or had complications due to their rapid growth. I think there is also just a certain number that don't make it naturally.
So if you can't find any reason and all your other fish do well, don't feel to bad, sometimes it just happens.