There are half pads called non-slip pads that work with saddle pads occasionally as well. They are basically the same thing as a gel pad just more designed for keeping your saddle and saddle pad from falling back and are much cheaper than a gel pad. I have had some luck with them in the past with a saddle pad, but it all varies.
If your saddle is sliding back, it may be too big for your horse. Get a saddle fitter in to help,check its in place and if it fails get a gel pad. It may be expensive but horses 🐴 aren't. Cheap anyway