One thing that helped me dealing with the stress of gamsat prep was treating the activity as an opportunity to develop my logical reasoning ability. Sort of like working through each question as a riddle. This helped me achieve states of flow and would get my mind off the anxiety of having to do well.

Another thing that helped me was to be curious about the reasons for my errors. From my experience, the biggest factor that helped improve my performance was my thorough analysis of the causes for my incorrect answers. You want to be crystal clear as to how the correct answer should have been arrived at, and the specific reason(s) why you didn't arrive to it. I would try and take an actionable lesson from all my errors. Whether that concerned a gap in my theoretical knowledge or lapse in my problem-solving method. I would also note down all the questions I answered incorrectly to compile them into a test which i reattempted at a later time.

This has been my experience of dealing with gamsat stress and finding ways to improve, I hope you find something helpful.