i got it last year, i was told by my dentist nit to put the gel in bottom of the tray but on the middle of the teeth on the tray, and only a tiny blob cos when u put he tray in ur mouth it presses the tiny blob over the individual teeth evenly. also, i have tried every percentage of gel, starting with 15, going to 25, and back to 10, and believe it or not the best one is he 10 percent. also i was told by my dentist to keep it in for at least 4 hours, preferably all night, but i soon discovered he keeping it in exacly 4 hours my teeth came up much brighter!
and yes it is sooo sore!! i used it everynight for a full 2 weeks till i was saticfied. couldn breathe with he pain! but it soon went away. sounds like ur using way too much gel, it shouldnt even be touching ur gums, my sis had this problem and its cos she used too much, try just putting a small blob on the middle of the tooth on the tray and c how it goes!!xx