I recently bought this car, and it has a fuel pressure pod gauge that isn't working. This is connected to the fuel rail (see pic).

Also, you can see the white wire in the background? It has a ring terminal on the end that looks like it fits on this thing.

Can someone explain this to me, how to get the fuel pressure gauge working again?

 

that is a pressure sensor. it needs the sender wire and a good ground. if either one is missing, the pressure gage won't work.
i would only use that setup to test fuel pressure only if the rubber hose is high pressure FI hose.
i would absolutely positively not use that setup to monitor fuel pressure on a regular basis. if and when the rubber hose either comes off, gets a hole rubbed in it, or blows out if it isn't high pressure hose, you will have a huge fuel leak.
with it close to the distributor, a single spark from a plug wire will result in a fire.
if it were my car, i would remove the hose and check the schrader valve for a leak and replace it if necessary, then put a cap on the fitting on the fuel rail.

 

Thanks - what type of sending unit should I look at getting for a permanent solution?

 

It's not the sender that's the issue. It's the way the sender is hooked up that can cause you problems. The sender should be installed so that it's not attached with a hose. Ideally, it should be screwed directly into the fuel rail.

The attachment might help. This is the instructions for Auto Meter electric pressure gauges. It may not be your brand of gauge, but it'll give you some guidance.