Fill with wood filler, sand stain. It comes in colors if you wish.
Depends on what a gash is. Do you have a picture?
I would use epoxy putty! https://www.charlestoncrafted.com/how-to-fix-gouges-in-wood-furniture/
Eileen: wood putty and then stain.
Is it a deep gash? Sometimes you cab use a damp cloth and an iron to fluff the spot up. If it's a deep or large gash wood putty is easiest, but it'll show through stain