Is there a way to put the whitening gel in my clear retainer tray and avoid contact with my gums?

I used the old NiteWhite 16% but now use DayWhite 9.5%. I went to use it and followed the instructions of brushing and flossing my teeth then applying a small amount in the appropriate places in the tray. But the second I put the tray in my mouth I felt a burning/bubbling sensation and took the tray off to find that the section of gum in between my top front teeth were white and irritated. I waited a good 30 minutes and was more careful but still burned my gums again. What am I doin wrong?