>>113248474You can hold your breath in space just fine. In fact, you can survive naked in space for a long time compared to, say, the bottom of the ocean. Factoring out the need to breathe, the next thing that kills you in space is either thirst or radiating off your excess heat. The latter takes several hours, if not days, assuming you aren't exposed to sunlight.
Of course, if you ARE exposed to sunlight in space, then burning/overheating becomes quite a large concern.
But all this is a moot point. All we'd have to do with the portal gun is get in a space suit, get in a vaccum chamber, and portal on up to the moon. Easy as pie.