When Busse and when Swamp Rat first released Resiprene C handled knives, there was some discussion about how a cut in the material could be fixed by pressing the two sides of the cut together and momentarily applying a lighter's flame. This same property could perhaps solve your dilemma. If the gap is small, perhaps put on an oven mitt, soften the Resiprene C with a flame, right behind the gap, and push the Resiprene C into position to close the gap. If the gap is larger, perhaps shave a very small piece off of another area of the handle, then shove it into the gap, and momentarily apply a flame, then push it down into where it belongs.