Any deck can be used by any player at any time, irregardless of skill level. It's the amount of analysis one has that determines the skill necessary. I.E., does a player pick up a deck and automatically know how it's supposed to run based on a quick look through? Or does the player pick up the deck and stumble through the cards. The more streamline a deck, obviously, the easier time anyone else will have trying to play it.
G/G is one of those decks that is just so streamline that people don't have to really know what they're doing to play it.
However, G/G is so streamline that they may miss the slight differentiations and little quirks of the situations at hand. Most G/G players for the first time ignore Sonic Blade altogether and go for the OHKO, which of course, costs them prizes that could be used later on in the game. The more skill one has, the easier it is for them to notice the slight changes, and the easier it is for them to win.
A player who has no clue what's going on may very well win a decent amount of games with G/G, but they're not going to be able to hold a candle to a good player with almost any other deck.
So, I agree - it doesn't take any skill to play, but it takes skill to play well.
Yeesh, long-winded reiteration, much?