[RED92LX50]

You need a 1/16" Male to 1/8" Female fitting. The 1/16" goes where the schrader valve is (remove the valve after you have released the fuel pressure) - and the gauge sender will screw into the 1/8" female end.

If you didn't get it when you bought the gauge (like me) you will spend more on shipping & handling than the cost of the part (like me). Unless you have a decent shop or HW store near you that carries them...