G-fiss

This is the legend about G-fiss. Let us start with a language-lesson:

A fiss? What is that?

It is Swedish (amongst others). It means "F sharp", that is: an F raised half a tone. In Swedish and many other languages -iss can be added to a tone, just like "sharp" is added in English.

And a G-fiss?

Well, that doesn't exist. It might have been an attempt to say "giss" (G sharp) and instead of adding "-iss", "- fiss" was added.

So what can it be then?

Maybe a G and a F sharp played at the same time. Why anyone would want to do that, is not clear.

It's also part of the chords Gmaj7 and Gmaj9.

And what does Google say about this?

Well, this is a G/F# (which is an F# chord with a G in the bass line).
For guitars:
http://tabs.bede.se/chords/index.php?chord=gfiss
(2 1x 0 0 3 3)