I fit's for a GS, none. Make a manometer.
Do a search, you should find plenty of info.
I bought a twin max and it's OK but not great. Shoulda saved my money and made a manometer
Manometer is the cheapest and always calibrated but....... can be time consuming
I have borrowed an assortment of different ones from mates, last time was a davida twing vacc gauges, nothing special but worked really well, nicely damped, just to check i swapped the gauges around and there was no difference in the reading.
I did use another sort, name slips my memory but i think it was TT tune ? two stainless rods in two tubes, a bit like a 'dry' manometer, beggered about for ages as one plunger was sticky, took ages.
I would buy the davida one, or similar