According to the GMAT handbook on mba.com, you can start writing only after the timed portion of the test has begun. That said, it's not 100 percent clear whether the "timed portion" of the test includes the timed instructions.
Best guess is that the rule is that you can't start writing until the test itself begins, but it could be that the rule includes the instructions.
In any case, it appears that some test centers don't bother enforcing this rule, while others do. So, whether you'll be allowed to write a cheat sheet while on the instructions is probably going to depend on which test center you're taking your test at.