Is this part of my truck?

[ESMD]

I was getting fuel (diesel) for my truck this afternoon. When I pulled the fuel nozzle out to return it to the pump, this was on the nozzle. It's a rubber washer that's u-shaped. I can't tell if it was part of the nozzle, my truck, or the fuel cap. It may even be part of someone else's truck! It the same size at the end of the fuel cap but does not appear to want to stay anywhere. Any help would be appreciated.

[avalonandl]

Don't recognize it.

[1st diesel truck]

What year is your truck?

The following link should be your fuel cap with a few pretty good images of a new cap.... Maybe the gasket you found fits over the center part of the cap as it looks like something should fit over the center white part of the cap? 

https://www.ebay.com/itm/OEM-NEW-Fuel-Tank-Gas-Cap-2015-Silverado-Sierra-2500-HD-3500-HD-DIESEL-/191766138415

 

Here's a good source of GM parts breakdowns using a 2015 Chevy 2500 HD fuel system to browse as an example, https://www.lookupgmparts.com/v-2015-chevrolet-silverado-2500-hd--high-country--6-6l-v8-diesel/fuel-system--fuel-system-components

https://www.lookupgmparts.com/v-2015-chevrolet-silverado-2500-hd--high-country--6-6l-v8-diesel

Maybe find your model just to see, https://www.lookupgmparts.com/