To fill a gap is to supply the thing that was missing. The evening class nobody offered, the shop the village had done without for years.
It takes in: fill a gap in the market, in the record, in your knowledge. The market one is close to a fixed phrase and is what a business plan claims about itself. A gap year is unrelated and is the year off between school and university, and stopgap is the adjective for something that will do until the real thing arrives.
1phrasefill a gapTo supply what was missing.
“The evening class was there to fill a gap in the town.”
fill a gap is an act of supplying what was missing.
Practised in Common Word Pairs