Hey, Welcome to the Forum!
Is it literally a hole in his gill?
Or is it more of a red cut looking injury towards the bottom of his gill?
I might be wrong because it is very hard to tell without a picture, but it sounds to me like his gill is "rotting" because of high Nitrates or Ammonia.
If you just put him in your tank, and he came with the injury, who's ever tank he came from may have bad water parameters.
I've found that it is pretty hard to find a fish store that really takes good care of their livestock. They are out there though.
As for precautions I would just recommend making sure you have NO ammonia and as little Nitrates as you possibly can.
Also check other water parameter (pH, alkalinity, phosphate, etc)
You should also feed him nutritious foods to promote healing.
If you could post a picture that would help a lot.
Like I said I might be wrong but that is that most likely thing I can think of.
Good luck,
Nick