I believe most pre and early war air gunners were either riggers of fitters, but there were also junior NCO crew members who were wireless operator/ air gunners.I think the pay for volunteer air gunners was sixpence a day.
Those in ground trades who flew on missions , lasting several hours, would often be made to attend to their ground duties on landing by over zealous NCOs.
When the NCO aircrew rank was introduced, many ground NCOs resented their new found status.
When the shooting started, many who had volunteered for air gunner duty, quietly went back to their ground trades.
Apparently the ground crew NCOs became more sympathetic to the new air gunner NCOs when casualties started occuring.
All that above was from memory from reading Max Hastings 'Bomber Command', many decades ago.