I asked them the same question last week, here's the response:
The Analytical Writing Assessment section of the GMAT® exam contains two
topics, each with a 30-minute time limit. Scoring of the essays evaluates
content and the writers’ reasoning. Examinees are not graded on spelling,
grammar, or typing ability. It must be in English.
So I take it it's fine to use non-American English.
Rule #76: No excuses. Play like a champion!