It’d repair slower than his hammer but it would help with when you place a turret really far back, or especially on high ground. Instead of having to walk all the way up, which can take a very long time depending on the map, or just never leaving the turret at all in order to repair it he can use his gun to do so. If he’s close to it his hammer heals faster and can upgrade so if you’re close it’s worth it to use the hammer over the gun.
It would allow him to be a bit more versatile as he does not have to ultra babysit his turret or just ignore it while it dies as he can do a bit of both while still having a presence in a fight himself.
ps: It will not upgrade the turret, only heal the turret.