Let's Establish the Parameters
There is no calculus on the GMAT. There is no trigonometry either. In fact, there isn't even any advanced algebra. The toughest algebra you'll ever have to solve is a quadratic equation. (Factor, don't use the Quadratic Equation.)
The GMAT curriculum includes four-function math, basic geometry, percent changes, rate problems, proportionality, combinations, permutations and probability. There is almost nothing in the GMAT problem solving section that isn't also on the SAT. That's why SAT math problems are good study material.
I know that people freak out over combinations, permutations and probability, but the truth is that even in this arena the curriculum is very limited and can be learned completely. (There is one formula for combinations and another for permutations. Memorize them.)
The actual math functions required on the GMAT are incredibly simple — so simple that, unlike the SAT, test takers aren't allowed to use a calculator.
Then why is it so difficult?
The GMAT math is designed to trick test takers. That's why very smart people who are un-coached on the exam often do poorly. They didn't suddenly get dumb, they just encountered a kind of math that wasn't taught in high school. GMAT math is not about orthodox mathematical manipulations; it's about reasoning your way through a logical mine field using inference and basic mathematical tools as your guide.
Should I Take a Math Class to Improve My GMAT Performance?
No. It's too broad. Some math classes can help, but most of what you would learn wouldn't be tested on the exam. Because the GMAT curriculum is so narrow, it's better to stick to GMAT-prep material only.
What Can I Practice With Other than GMAT Material?
Do math problems from the SAT exam. Buy the SAT Official Guide and do the math sections. There are 10 full SAT exams in that book along with a math review section. You can get the book at Amazon. It's probably the best starting point for GMAT test takers looking to improve in math fundamentals — better even that the GMAT Official Guide.