I fitted a GSF tank to an old 1 litre mk3 i had. as above, had to bend the filler neck into a better position to get it in. then it was really tight getting the supporting straps round the thing. I was never happy with that tank, everytime i shut the boot after that, there was a vibrating/rattling noise.
I know the vw ones are expensive, but i would seriously consider it if/when i have to to one on my G40.