No, not a gadget, more a different kind of set-up to combat the compromise employed with most fretted instruments. The idea is to give a more pleasing-to-the-ear sounding third when playing a G or E chord, for example.
Speaking of set-ups, PLEK is getting some good reports but at £120 a time it seems an expensive way to squeeze a little extra out of your instrument.
But that's probably true of all the systems discussed here.