Actually the term "Gummer" is a throw back to the age 60 retirement days, when there was a "special unit" of the training department that did all of the test flying and no-revenue repositioning flights, where a maintenance issue(s) precluded the use of the aircraft for service. They did all of the ferry permit flights, engine inop takeoffs, re-rig return to service flights etc...

Tightly held, "Gummers" were hand chosen, older than age 60 guys, that got a full line of time pay (as a consultant...most had taken their lump sums) for being available at the drop of a hat to go fetch broken planes and the like. To make it into the "Gummer Squadron" you had to "know somebody".