No, gas grenades are not incendiary, they just use the same damage model. Damage type and damage calculations can vary between weapons. The most typical example is napalm vs incendiary, where one actually works off its power rating and the other only uses it to calculate the damage radius. Gas grenades are the choke/smoke version of that.
There's only one type of choke, but it can be applied in different ways.
Reptoids take 80% damage from choke, regular enemies take somewhere around 400%. So essentially they have 20% choke resistance, there's just no good way to communicate this hack to the player.
5-10 * 80% is 3-8 damage, so they can eat at least 5 gas grenades and more likely 7-8. But you can just shoot them as normal if they're wearing suits.