OK...it's a piece of rubber that you can buy from many... BUT, it's about the cheapset, works fine and arrived very quickly and is as good as any! Tip: as the gasket is sometimes very hard to lever off, put a drop of olive oil on it before you put it back on!
I fitted this to my Gaggia Baby espresso machine as the original one had gone hard and was leaking when the group was fitted.
The fit was perfect and it solved the problem for a few months.
The original lasted a few years, this lasted a few months.