This is an interesting debate to have with many facets involved. As a Canadian, we tend to use British English more than American English (think neighbour vs neighbor), but both are acceptable uses of the English language. As Karishma pointed out, language is constantly evolving, just as Shakespearian English is no longer the common vernacular (seriously have you tried reading through that?
Thou doth noteth understandeth the first timeth around), English in 200 years will have probably changed significantly.
Most modifications seem to be about being more concise, such as the oft used "drive thru" now, or about a slang modification to the word being used. In the context of GMAT (or TOEFL, I presume), a little slang or British/English spelling won't affect you on the AWA, but hopefully you're solving sentence correction problems with the correct rules of grammar and not what you heard from Timbaland's CD (the way I are?).
Living in a bilingual province (French and English), the amount of English, French and Frenglish I've heard at different levels is staggering. French people routinely talk about their "bosses" and "jobs" and where they did their "shopping" on the "weekend", even though none of these are French words. It goes both ways, of course, as English people here talk about "fait accompli" and "digestif", French words that everyone seems to understand in English for lack of a better translation. In the end, language is about communication, so if the other person can understand your point, you've done a good enough job communicating to them.
The GMAT even highlights this point in the other direction as well. Some Reading Comprehension passages are so dense and difficult to read that no one understands what they are trying to say. Using a vocabulary that is correct but no one understands is just as bad as using slangs and abbreviations that no one can follow. In either case, the meaning is lost through the choice of medium. As the internet meme goes: ain't nobody got time for that!