I is a
subject pronoun while
me is an
object pronoun. Hence, the usage depends on whether the intent is to use a pronoun as a
subject or as an
object.
Having said that, chances are
minimal that you would specifically be tested on
I vs me, since almost all sentences on GMAT are in
third person (
they, them, he, she etc).
Our book
EducationAisle Sentence Correction Nirvana discusses
Subject and Object pronouns, their application and examples in significant detail. If you or someone is interested, PM me your email-id; I can mail the corresponding section.