Gambit's voes are going to be partially solved in the next Destiny 2 update as Eriana's Vow will get its regular damage back.
Gambit in Destiny 2 has always had its issues, mostly revolving around the access to special and heavy ammo that was intensely used by invaders who also have a legal wallhack for the duration, leaving the defending team at a severe disadvantage. The latest weapon-related problem has roots in a bug, however, as Eriana's Vow became unintentionally overpowered.
The hand cannon takes special ammo but it's not meant to be a one-hit kill unless some damage boost is active, and yet, that's exactly what it's been doing since The Witch Queen release. To make matters worse, it doesn't even require a headshot as a simple body shot takes a Guardian down and only a full overshield can save them, in which case they get a sliver of health left. Headshots still down them in one tap.
Anyway, the fix for this issue is coming on March 22, 2022. Since this is a Tuesday, it's safe to assume this will happen at weekly reset, meaning 10:00 AM PT / 6:00 PM CET.
What caused this bug in the first place remains a mystery but the players speculate it's jus that modifiers between fighting AI enemies and other players didn't happen so we kept dealing hundreds of damage even to Guardians. Still, the numbers look a bit high.
Either way, you have a few more days to abuse the bug and then it's back to normal - killing people with Gjallarhorn, Truth and perhaps Xenophage.