Depends on what your regulating authority will allow since it is not detailed in the build plans.
If you do do it, keep in mind that the stabilator is a balanced control surface.
Adding weight to it in the wrong place is not a good thing.
When just filler is used, cracking in the future is pretty common regardless of what type unless the gap is bridged with fiberglass which is even a worse choice because of the balance issue already mentioned.
Your airplane will be covered with skin lap joints and other types of structural intersections. Once it is painted, I doubt you would ever be bothered by the gap that currently exists.
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Van's Aircraft Engineering Prototype Shop Manager
RV-6A (aka "Junkyard Special ")