Besides the gun manufacturers lobby and Saudi Arabia, of course.
The treaty is meant to block gun sales to drug cartels, war criminals and terrorists but Donald says it's an attack on the Second Amendment and American sovereignty. Seems like that's just the opposite of what's in the country's best interest. And no, I don't see any connection to the Second Amendment. Nothing in there about exporting weapons or facilitating war crimes and genocide.
Don't look at me, I just work here.