When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
It tells you what your fuel pressure is.
You want a gauge that has a full scale deflection of 100psi. It is recommended that you get an electronic gauge as opposed to a mechanical gauge if you plan to have the gauge inside the cab.
The recommended port to connect to is the post-filter port on the fuel bowl. It is the lower right Fuel Plug in this image.
Assuming your pressure sensor has 1/8 NPT threads, you will need this adapter: Ford 7.3L Diesel Powerstroke Fuel Pressure Adapter straight superduty
In addition to telling you about your pump as you correctly said, when the sensor is mounted post-filter it can tell you of your filter is plugged. If/When you have larger injectors it will tell you if your fuel system is keeping up with demand.