I use PolarPro and BlurFix air, without balancing the gimbal. So far, so good.
PolarPro filters are a fraction lighter.
I recommend an ND4, ND8, and ND16.
If you will fly around water, snow, or reflective surfaces, getting and ND/PL combo is a good idea. However, PL filters have to be rotated to the correct angle to get the benefit of reflection reduction. (Tip: if you have Peau replacement lens, DO NOT rotate your PL filters while they are on the GoPro - this can defocus your lens. Remove, rotate, then slide straight back on, without twisting!).