That is a stock gas cap for a 1971-on Triumph Spitfire, GT6, and I believe the Stag and 2000. So it is actually curved to match the rear quarter of those 4 Models.
The Triumph Part No. is 718750. The problem will be it hasn't been manufactured in a long time.
www.Spitbits.com shows the part but looks like not in stock.
Your bests bet for a new one will be
www.rimmerbros.com in the UK. I hear they are remanufacturing them.
You should be able to get the gasket a lot easier.
I need to get back to work but if you search for Triumph part number 718750 you will find other sources for used ones and possible NOS.
Good Luck,
Richard