If it is the OEM boost gauge, it comes with the proper T-fitting. If it's an aftermarket one, just measure the inner diameter of the tube that came with it and then slide off the recirculating valve tube and measure that ID.
I don't remember exactly but the sizes were roughly 3/8" for the recirc tube and 5/16" or 1/4" for the line to the gauge. I made my own T-fitting out of the brass fittings from Lowes since I bought mine used and the T-fitting was no included. I just tucked it away below the intercooler so it looks stock/OEM with a quick look.