Steel Adapter, 1/4-18 Male NPT Run To 1/4-18 Female NPT Branch, Tee

That's exactly what this fitting is built for. The run ports are both 1/4-18 male NPT, so the main line passes straight through, and the branch port is 1/4-18 female NPT for your side connection. Steel construction keeps it solid in hydraulic line work where brass wouldn't hold up as well. As long as your thread sizes match up on all three connections, this handles that split without interrupting the run.

For adding a pressure gauge, this tee adapter should do the trick. Its 1/4-18 NPTF connections should allow for a solid, leak-free connection in your hydraulic system.

Function-wise it is a tee adapter with a 1/4-18 male NPT run and a 1/4-18 female NPT branch, so it’s meant for splitting a line where the straight-through side is male and the side port is female.