As Chetan mentions, since GMAT is adaptive, the level of difficulty of probability would depend on how you are performing in the exam; if you are performing well, the level of difficulty would be a bit higher; if you are not, the level would be easily manageable.
For that reason, even if Probability is not your strong suite, we suggest that you don't skip this topic altogether, so that even with basic understanding of this topic, you can tackle at least
simple Probability questions on the exam.
Do remember that getting easy questions wrong, attracts significant penalty on GMAT and you would not want to be in that position.