I think they are electric. The back of the gauge only has 2 wires, and no vacuum nipple. I think it's specific only to S15's too. You can use the holder in S14's though, but you have to cut the top bit of the plastic to fit.
I've just located a GTT for sale at one of the used/import cars dealer opposite Assuntha school in pj, after viewing the car, it looked good. Its unreg car manufactured'04 (although we knw its last...