Why does my 18 week jack russel puppy keep gagging

WE DON'T KNOW!
Perhaps something caught in his throat such as a toy or a bone. We don't know what you feed him or leave lying around or how much you watch him, or what shots he's had or how much he is outdoors or whether he is eating and drinking and sleeping, on and on...
So take him to the vet.

Could be a trakia problem