The Infinity Gauntlet is unable to kill anything, being just a piece of metal. Maybe if you hit someone on the head with it, ok, but it's not an efficient way to do it.
The six Infinity Gems instead, as explained in many occasions, give their owner control over fundamental aspects of the universe making him the equivalent of a god. And it's nowhere implied that the Gems themselves are some kind of AIs or grant wishing magic, so there is no
How do the Infinity Gems kill you?
The gems do nothing. Its owner uses its new powers of control over every aspect of the universe to do whatever he pleases, however he pleases. Thanos may have decided
- that all those peoples will never exist anymore
- to physically disintegrate them
- that they never existed at all and then rewrite all the universe memories so that we all remember they existed, just for fun
- he may have killed all the people he intended to save, created a brand new reality and moved their consciousness there, leaving the other ones in a doomed universe lacking resources.
- he could have taken a different decision for each and every one of them, "killing" each one with a different method.
- he may have started snapping his fingers, then go to a dancing course, teleport to every restaurant in the universe, tried every possible food ever cooked, then he may have spent billions of years travelling across time and space to solve every unsolved mystery, then decided it was useless because being a god he already knows everything, so he undo everything, than he teleported near every person he decided to kill, punched them in the face instakilling them, and finally finished snapping his fingers.
He is god, he could do everything and then undo it and redo it over and over again, and we still wouldn't know anything about it.