Math has actually been one of the hardest topics for finding quality units, in my opinion. Many units don't focus on problem-solving, apply and evaluating mathematical concepts, or creating using math. These units also tend to cost more than the average unit.
One of the best free resources I have found have been these problem solving decks for grades 1-8. They are quality problems, often with multiple correct solutions. They can be utilized in the regular or gifted classroom, alone or in pairs, and are definitely challenging!
My other favorite resources for math are Open Middle, NRICH (amazing resource, but the organization takes a bit of getting used to), and the University of Waterloo (especially the Problem of the Week for grades 3-12).
You could absolutely do some great math in your gifted classroom with any of these resources, but if you prefer a more cohesive unit, check out the grade levels tabs under math or click below.