Now I know I shouldn't expect much from the LoL community, and I guess I've been spoiled by SC2 but do people in LoL not know you shouldn't gg until the game is over and its considered rude to do so?

With the release of Dominion, offensive GGing has become much more apparent Don't offensive GG, it makes you look like an ass.

I concur, this happens ALOT. Today I had a game where the team thought they won did a GG before anyone on our team did... Then proceeded to get pushed back 3 more times saying GG everytime they thought they won. Eventually they did win but it was still very unsportsmanlike.

LOL is a pretty immature community of players, I've tried to explain offensive gg to the other team respectfully so they know because maybe they don't know. Only to get told "gg noob, you suck".

Offensive GG's are great because when the offending team loses you get to gloat all the more enthusiastically.

because people think they have it in the bag when its like 300-80 or something, then realize you cant win if you dont have majority of the points

It's what we like to call gamer's etiquette.

You don't say GG until the game is done.
As the victor you wait for the other team to say GG first. If they don't say it, then you don't say it.

Many games follow through with this very well. League does not.

Enemy: GG now surrender
Teammate: /surrender
Me: Can't do it... can't because he told me to.

I'm pretty sure that it's because people don't realize that you only tick off their nexus when you hold a majority, unlike other similar capture the point type maps they may be familiar with *cough*AB*cough*.
Thus when they're way ahead they think it really is over

So far about half my games seem to have me say GG as the Nexus is exploding, followed by "STFU"s and insults by the enemy team, whether we've lost or won.

Point being: You can't say GG no matter what without morons being whiney *****es about it.