As someone who sat the GAMSAT last year, I would agree that doing CHM1011 and 1022 is definitely helpful for section 3. I didn't do chem as an elective myself, but I sought lecture notes from people who did do it, and have heard that it helped familiarise them with some of the section 3 questions, especially with the amount of organic chem in the exam.
As helpful as it is, I would say it's not a must, meaning if you had other electives that you really wanted to do, it wouldn't hurt to do those instead (I ended up picking maths). The crux of completing the science section is to be able to develop a logical answer based on the information given (which will come from a range of biology, Chemistry, and physics questions). This means in order to study for the GAMSAT, the most important thing to do (in my opinion), is to get familiar with the style of questions through practice exams, rather than worrying about learning every topic (this should be your second priority if you are struggling with a particular type of question).
I hope this helps!