It's excellent that it seems like you're studying for the
GMAT proactively! You'd be good to start with a refresher since you haven't utilized pre-calculus and algebra in a while. It's true that Khan Academy is an excellent tool for reviewing math concepts.
To determine which particular math areas to concentrate on for the GMAT, I suggest looking over the content outline or syllabus. This will help you understand the mathematical ideas that will be assessed on the test. After you have that list, you may start with the subjects you feel most out of touch with or are rusty on by comparing it to the Khan Academy course offers.
Manya - Princeton Review is a great option for supplementary materials. They provide thorough GMAT preparation programs that address the test's verbal and quantitative components. Their knowledgeable instructors will help you navigate the content and offer tailored support to strengthen any areas where you may be weak.
Regarding a study schedule, I advise setting out regular, consistent time to review math ideas and work on problem-solving strategies. You'll gradually gain confidence and skill if you set aside a few hours each week to work through practice problems and go over important concepts.
Regaining familiarity with math ideas requires patience and consistency, so keep that in mind. When the time comes, you'll be ready to take the GMAT if you're dedicated to it and have access to the necessary materials.