Hi Scon.
If you can believe it, after taking the GMAT multiple times, hitting my score goal, and coaching people who are preparing for the GMAT for years, I'm still capable of freezing up when I first see a tough question. The key thing I have found is that freezing up is not the problem. The problem is what one does next.
If you freak out because you are freezing up, then the clock is going go be ticking and you are going to lose time.
So, part of the solution to what you are experiencing is to respond differently to freezing up. Become self aware, realize that you are freezing up, and make it your game to see how fast you can get going again. Many people have to prepare for the GMAT by learning how to handle quant concepts. That's their path to winning the game. For you the path may be different. You may have a different game, learning to unfreeze and get to work. It's still a game; you still have a skill to develop.
I agree with other posters that exposure therapy is another part of the solution. Become more accustomed to working with timers. Make working under timed conditions your hobby. Make it a comfort zone for you.
Another part of the solution is to be rational. If you can answer the questions in under two minutes each, then really the timer should make no difference.
In fact, here's an idea. Take a practice test but don't look at the timer. Maybe even cover the timer with a piece of tape, and just answer as many questions as you can until the timer runs out. See how you score.
Alternatively, turn the tables on the timer. Start a timer and see how fast you can answer questions. I mean, rather than seek to answer them in two minutes each, as if the timer is in control, see just how fast you can solve them. If you can solve them in an average of 1:30 each, you are set. If, on average, you take, for instance, 2:30 or more for each, then you have more work to do, and the timer is only part of the issue.
Finally, while you are strong in math, I bet you could benefit from continuing to develop your GMAT specific skills. The better you are at answering GMAT questions, the less jitters related to the timer are going to matter.
Overall, just see this timer issue as another part of the game, a game you can learn to win.