There are a few different ways that you can rubberize a gun grip, depending on what type of grip you have and what materials you have available. One option is to purchase a pre-made rubber grip wrap or cover that can be slid over the existing gun grip. Another is to use liquid latex or another type of flexible adhesive to coat the gun grip and create a rubberized surface.
If you want to add some extra traction or texture to the grip, you can also add some sandpaper or other abrasive material to the coating before it dries. This will give the finished product a little bit more of a gripping surface.
Experiment with different methods and find one that works best for you and your needs!