clean out any dirt / sand
Clean out with thinners (can never remember the right one)
rough up the surface (where the gel is missing) so the gel will have something to key into
Apply gelcoat / gelcoat filler and then cover with baking paper (grease prof paper) and then allow to cure.

File / sand and polish and job done.

Something like that is a small looking chip - should only take a few minutes to fix (depending on ambient temp).