Can we hide the timer in GMAT?

Some test takers hate the timer, others need to know how long is left and how their pacing is going. You can certainly hide the timer, but the best approach I've found is to check back every 5 questions to make sure your pacing isn't way off. Most often, for the quant you need to plan ~10 minutes for every 5 questions, give or take 20% either way. If you've done 10 questions in 30 minutes, you'll likely run out of time and feel crunched at the end.

A lot of people dislike that little ticking box, but it's important to be aware of it. As Vercules indicated, the timer will reappear with 5 minutes remaining, but keep an eye on it before that threshold as well to avoid any panicky surprises.

Hope this helps!
-Ron