Yes it's a Kaplan book and yes I teach for Kaplan, but I can honestly say that over 7+ years of teaching GMAT prep and my students using a variety of books, those who are struggling a lot with math fundamentals and content rave the most about Kaplan's GMAT Math Workbook.

What they like about it, and I can see why, is that it take a concept, let's say solving simultaneous equations. It first illustrates how to do that concept. Then it has you do it many times, with say 20 drills that just have you practice that concept (perhaps as you learned math in high school). Then it moves on, still on that same concept, and puts it into GMAT-style practice questions. So it's one of the resources that reallly drills the most on each particular concept, from content to practice questions for each.

When I've suggested that book to my students over the years who are struggling, they often come back and say that it was extremely helpful. (Note that as of this year, any GMAT student enrolled in our classroom or full online courses actually does get this exact material; that was not the case in previous years, so my students were sometimes supplementing with this book and others). Open to thoughts from any on the boards who have used this book, but anyway that is what one that I hear about from my students and would support.

Good luck!