I love gaffer tape for some stuff, but not this. Gaffer tape does not typically stick well to it's own backside - like when you are wrapping it multiple times and it is overlapping itself. I believe that is due to a silicone on the top side of good gaf tape that helps make it come of the roll easily, especially as it ages.
You could french whip it, starting the whip with a constrictor knot. You might want to apply the whipping over a wrap of rubbery material, and allow it to overhang the whip, so the techflex has some strain relief at the edge.
Alternatively, a strip of rubber from an inner tube, wrapped and then cleanly superglued, would be easy to do and look clean. You might try using contact cement, but I think that would be more difficult. If you end the tip to a narrow section, you could weave it back underitself and have very little seam to deal with.