Is farming gauntlet bosses an exploit?
No. Farming is intended. Anet is fully aware of the farming: https://forum-en.gw2archive.eu/forum/game/bltc/Economy-concerns/first
Plus they’ve had two patches so far. They haven’t implemented diminishing returns or time gated stuff.
If you’re not farming the gauntlet, you’re losing out. This chance won’t come again.
I don’t see how it’s an exploit. You’re defeating them using your own skill. It’s not the players fault that they’re easily farmed using the right specs and setup. The staff have tried to remedy the issue but failed to change much. I suspect it will get nerfed again sometime in the future, but it’s certainly not an exploit.