The gap you’re describing sounds like it’s a fascia problem. Possibly caused by wood rot. It depends on the contractor as to whether this should’ve been included in the roof job. But, if it wasn’t pointed out or in the wording of a contract… You might just need a handyman to come out.
If the fascia behind the guttering is really bad, replace it. If the fascia is still in good shape, you could do a patch with some mesh and quick set, then paint it. That’s kind of cheating, tho, if you’re just gonna sell the house. If the fascia’s covered by siding, maybe it’s just loose.
Bottom line? Talk to the contractor about your concerns, ask him if he can do anything about the gap. You do have a new roof. That is a good selling point. Pay the man for the work. But, next time… get a contract.