I recently bought this car and noticed gasoline slowly oozing out of this piece. I don't know what it's called, but I believe it's missing a cap of some sort. If I knew what it was called, I would get the part I'm missing.

 

You need a new Schrader valve. Fuel shouldn't be leaking from there, cap or not.

 

You might be able to tighten the schrader valve. There is a plastic cap, but it just keeps the dust out, not the fuel pressure in.

 

Try and snug up that valve before getting a new one. A tire shop will have the tool or you can buy one cheap from a parts store.
I have a crap load of those caps.
Early cars had aluminum and SD cars got the black plastic, from what I have seen.
I picked up most of my spares at a JunkYard from front drive V6 Jbody cars.

 

I'll try to tighten the valve a bit and see if that fixes the problem. I just noticed the spiraling grooves and immediately assumed something essential was missing. Thanks for your help, guys.

 

It's not the outside part you'd need to try to tighten, it's the core inside.

 

that makes sense. I was wondering why it was suggested that I go to a tire shop for the tool.