What can I do to expedite hair growth in the gap or the surrounding area?

Growing hair quickly in that gap isn't feasible, but you can consider a hair transplant for forehead reduction, addressing a receding hairline, and correcting asymmetry in your frontal hairline/forelock. I recommend considering an FUE (Follicular Unit Extraction) procedure, as it can achieve your desired hairline in a single session through dense packing hair transplantation.