Short answers, pulled from the story.
The Mid-Atlantic gap was an area of the Atlantic Ocean that lay beyond the range of land-based RAF Coastal Command antisubmarine aircraft during the Battle of the Atlantic. Merchant ships crossing this zone had no air cover, leaving them exposed to German U-boat attacks. The gap was finally closed in May 1943.
The Mid-Atlantic gap was closed in May 1943, when RCAF Very Long Range Liberators became operational in Newfoundland. By that time, the Battle of the Atlantic was largely won.
The gap was closed primarily by the Very Long Range Liberator, known as the VLR, an American-built aircraft operated by RAF Coastal Command and the Royal Canadian Air Force. As late as early 1943, only thirty-eight VLRs were covering the Mid-Atlantic Gap.
Coastal Command consistently received lower priority for long-range aircraft than RAF Bomber Command. By mid-1942 it held only two squadrons of Liberators and B-17s combined, and Arthur Harris sought to redirect even those to bomb German cities whenever Coastal Command showed signs of success against U-boats.
The Leigh light was a powerful searchlight fitted to large aircraft like the Wellington and Liberator. It entered service in June 1941 and was used in combination with radar to illuminate surfaced U-boats at night, when submarines were most vulnerable while recharging their batteries.
Germany developed the Metox receiver to detect ASV radar pulses before aircraft could spot a submarine, and after capturing an H2S radar from a downed Stirling Pathfinder in February 1943, Telefunken produced the Rotterdam Gerat. The FuMB7 Naxos U followed, but it entered service the same day that the H2X radar, which Naxos could not detect, became operational in Coastal Command.