You don’t have to gag them. Just press your finger sideways into their back teeth so they can’t close the bite. They’ll immediately try and push out your finger with their tongue. It stops being a game for them real quick.

But this is also a last resort. They’re are better techniques as discussed in other comments. The earlier the intervention the easier they learn.

In my area it’s become lore that cats only biting becuase they didn’t grow up with other kittens to give them social feedback for bites. The the point where the shelters will insist kittens be adopted in pairs.

It seems like an insanely lazy approach to me. Like getting two dogs so they’ll play without other and leave you alone.