There is nothing wrong with guessing on the GMAT. You aren't there to prove you can solve problems by traditional methods. Your aim is to select as many right answers as possible within the allotted time. Nobody will see your calculations; nobody cares how you found your answer. The only things that matter are whether your answer choice is correct and whether found it in time.
Quadratic equations, like any other arithmetic topic, have specific importance in GMAT. You can expect to see stand-alone quadratic equation problems and quadratic equation questions combined in problems with other topics such as number theory or progressions .