GMAT prep classes can be very effective, but be careful about which one you select--the quality does vary among the different companies. The test prep company I usually recommend is the Manhattan GMAT course (virtual or classroom), they have an outstanding student performance record. As a Beat The GMAT member, you are entitled to a 10% discount on all their classes and products, go to https://www.beatthegmat.com/discounts.html for more info.

But before committing to a course, also think about self study as an option. This community mainly consists of self studiers and there are a lot of people who do very well on the test on their own. The consistent key to success is setting up a solid study strategy at the beginning and staying disciplined with practice. For an example of what one study strategy could look like, please visit https://beatthegmat.blocked/. Also, check out the self study resources available at https://del.icio.us/beatthegmat

I wouldn't worry too much about the math--you may be rusty now, but you'll be surprised by how quickly you'll remember these old concepts. I was in the same boat as you were in during my prep, and being the nerd that I am, I actually found doing these math problems kinda fun.

A final word of advice--I think that 6 months is too much time to study for the test. Yes, there is such a thing as taking too much time to study. I usually recommend 2-3 months of prep. Beyond that, you start to run out of quality practice material/tests and you even start forgetting concepts you learned early on!

Good luck!