I think I have seen projects using miniature neon indicator lamps tubes as GDD's / mixers

I have often toyed with the idea of making oscilloscopes, and spectrum analyzers with them, but that would take a long time to explain...

Also, I miswrote it's Glow Discharge Detector

Another idea is repurpousing plasma display panels as a grid of GDD detectors, as an imaging array.

For an example circuit using a neon indicator lamp see figure 1 of the 2010 article:

http://www.academia.edu/download/44461415/Heterodyne_Detecti...

This 2016 article measures the light emitted from the neon indicator lamp instead (with FMCW scheme to detect distance as well)

https://www.osapublishing.org/viewmedia.cfm?uri=prj-4-6-306&...

This setup improves response time (bandwidth) of the output signal.