G fiss doesn't exist. A "fiss" (f sharp) is an f raised half a tone. Find the black key to the right of any f in a keyboard instrument, and you've got the f sharp.

Where in english you add the word "sharp" to denote this, in Norwegian, Swedish and god knows which other languages, you add the suffix "-iss". Thus, you can have a "giss" - g sharp - which is a g raised half a tone. This works on all of them. Aiss, diss, ciss, etc.

The reverse, lowering by half a tone, is in English called "flat" (g flat, f.ex), and in Norwegian and (probably) Swedish called -ess (dess, gess, ass...).

There's no "fess", lowering the f will give you an e. (Same as a "hiss" or "biss" - a raised h/b - will be a c)

Makes sense? I've never actually tried to explain this before. :P