First of all, don't feel bad because of the GAT no matter haw badly you did, because it's nothing. At my school, tons of people who eventually achieved ATAR scores of 95+ left early during the GAT, me included. The GAT has 3 sections (up tp my year at least), Writing (2 essays), MC on Humanities, and MC on Science/Maths. Because the GAT is to test students' general achievement and the questions are really not hard, if you have interest in a particular area, you can answer the questions. For me, I only care for Science/Maths, I got standardised score of 50 on that, and I got something like 20~30 on the Humanities section, because I genuinely don't care. But trust me, it won't reflect your intellectual capability or reflect on your 3/4s whatsoever.
Good luck.