This is going to fully cement my position as a gaper, but I believe it could come from the gap that a new skier may have between their goggles and toque/helmet/hat. An experienced skier will know that this gap will lead to frostbite and or an icecream headache. The 'gaper gap' some might say.
It is generally used to refer in a pejorative fashion to those who are worse skiers than you. A prime example of a gaper is someone who rarely skis and still insists on riding the lift to the top and then sideslips their way down the whole mountain. Or those guys who straightline everything and can't stop. Or people who can't carve. Or people who can't ski bumps, or people who can't ski powder. Or people who ski their big 100+mm powder skis on groomers. Or people who ski 210 straight skinny boards.... etc. Basically anyone who attacks the mountain in a way you don't agree with.