David L. answered 05/29/20
Expert, Gentle GRE Quant, ACT Math, and SAT Math Tutor
The Educational Testing Service is the author of the GRE Math Subject Test. Take a look at their web page for the GRE Math Subject Test at https://www.ets.org/gre/subject/about/content/mathematics. It says that 50% of the test's content is related to calculus. The fact that you have never taken a course in multivariable calculus is your biggest issue in doing well on this test. At the bottom of web page above is a link to this document: https://www.ets.org/s/gre/pdf/practice_book_math.pdf. Download it, and take the enclosed test under realistic testing conditions.
So you need to cover the material of typical courses in multivariable calculus, ASAP. If you're still taking college classes as an undergraduate, sign up for the courses in multivariable calculus that you missed.
In graduate school, your income will probably come from your salary as a Teaching Assistant for various classes, including calculus classes, and if you cannot be a T.A. for multivariable calculus, that makes it more likely you will sometimes have to be a T.A. for remedial math classes, which you may not want to do.
Good luck!