Fuel pressure

You need the special adapter that goes between the supply line and the fuel rail so the gauge can tap into the line, it will be in most pressure test kits. This guide can help

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Do not try cutting the line and tapping in, it will leak and probably fail. If the pressure test kit you have does not have it check the parts stores for them, they are a common fitting. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.

Most Auto zone and advance auto have fuel pressure testers they loan out with a refundable deposit that as all the fittings for most cars and trucks.

As MH says, the loan a tool kits will have a bunch of adapter fittings and lines to make them work. The one you want will plug into the factory fitting for the fuel rail.