If you can try to put some topical antibiotic cream on the affected area so it doesn't get infected.

If the injury is as bad as you describe, you might consider seeing a vet. If you can, attach a picture so maybe we can help you with a decision to see a vet.

The skin will grow back, but, for grandis the new skin will be noticeably different than his regular skin.

I had a juvenile standing's day gecko that got into a fight with his older brother and a big patch of skin was ripped. It healed up nice, and luckily with standing's you really can't tell he was ever injured.
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Lisa

1.0.0 P.m. grandis (Linus)
4.3.0 P. standingi (Toby & Kona and their 5 babies)