For some instruments, it's the same skill; you play an electric guitar similarly to an acoustic guitar, or an electronic organ like any other keyboard instrument. The differences can be covered by familiarity penalties, for which GURPS already has rules.
Where I needed new specializations was in instruments where the basic movements were entirely novel—for example, the theremin, which you play by waving your hands at two antennae. But it turned out there were several different control interfaces! I originally had Gestural, for instruments that worked by body capacitance like the theremin or the ondes Martenot, and Tactile, for instruments that worked by touching contacts like the trautonium. But one of my playtesters pointed out the stylophone, where you touch an electrically active stylus to contacts. (Interestingly, it was used on Bowie's "Space Oddity.") Was this a different skill? It seemed sort of akin to Keyboard; but it's not identical, and I had Tuned Percussion for instruments like the xylophone and the glockenspiel, rather than calling them Keyboard. And it was different enough from the trautonium (where you slide your finger along a wire, pressing it against a metal plate) so that I didn't think they could be subsumed in the same category. But what to call this one, if not Tactile?
One option was just to use the names of the instruments—Theremin, Trautonium, Stylophone—the way I used Lute or Horn. But most of these names are seriously unfamiliar! Another, which I think I may settle on, was to describe the control input on the device: theremins are Antenna-Based, trautonia are Slider-Based, and stylophones are Contact-Based or Stylus-Based. On the other hand, I could use the instrument names and put the control surface into the short definition.