HELP!!!

My female shtct leopard gecko has gotten darker in color and shes not eating. Also she opens her mouth and gags. I haven't seen here eating the sand but im worried she may be sick please help.

It does sound like your gal is ailing.
It can be difficult to diagnose what is wrong via posts/email &/or limited info. Could be a respiratory infection, something caught in throat, parasite load, etc., etc., or even impaction.
The best course of action is to make sure husbandry is correct, see if you can get her to drink some water (which should always be available anyway), and, certainly, get her to a vet for proper diagnosis and treatment.
Also, get her off of sand and, instead, use paper towels, newspaper, or other non-toxic "substrate" that is not loose substrate.
Meanwhile, just in case there is an impaction, you could also try giving her a warm soak, and a drop of mineral oil, to see if the impaction can be passed. This would not hurt anything even if impaction is not the problem.
Again, this gecko needs to be seen by a vet.