What is a Gimmick Capacitor?- and Answer

Basically a gimmick capacitor is a very low capacitance capacitor.

Think of a "fixed trimmer" capacitor. Dirt cheap and easy to make.

 Typically it is 2 pieces of wire twisted around each other, but I have seen pieces of twisted bell wire, hookup wire, coax cable section, lamp cord or speaker wire used.

 One changed the capacitance by twisting the turns tighter or looser, or shortening or lengthening the parallel wires in lamp cord or coax.

Adjusting was done on the parallel wire (Zip or speaker wire) types by cutting the wire shorter & shorter until the freq. just started shifting the wrong way, at which point one stopped cutting. In reality, this could be as fussy as 1MM removed per cut. 

Capacitance is typically in the very low PF range, from a fraction of a PF to maybe 5 pf (?) max.

Mostly used in tuned circuits or oscillator circuits to tweak them to the last gnats hair of accuracy.

(Note: These are not to be confused with the short pieces of parallel wires that looked like super small zip cord used in some models of color TV sets on the CRT socket to act as a spark gap in case the CRT arced over in the base.)

I have seen a gimmick capacitor used as a trimmer capacitor on a variable radio tuning capacitor also. IIRC I also saw one used in a TV tuner decades back too, but this is an extremely rare occurrence.

They were also used in many home brew Ham radio circuits to change the freq. of a xtal oscillator in a transmitter.