I don’t really make gums for my characters, as I usually go for a cartoony look.
I suppose what you could do is make a cube extruded many times across, so that there is one cube over every tooth, if you understand.
Then subdivide the gum. Grab the vertex that is over the middle of each tooth and move it up vertically away from the teeth. Smooth it and subsurf it and it kinda looks like a simple gum. Pull a few vertices around to make it look lumpy and play with the textures. After about 1 minute, I’ve come up with this.
Hope that helps. Other people may be able to make far more realistic looking gums though.