To see the new export options you need to enable beta updates and restart Spine.
The old gif export had this flickering issue:

But the new has even beautiful crisp edges (without the white artifacts)

If you're still aiming for more, you could also try out the newly added APNG export that has more colors and the best alpha, but might be heavier, or export a PSD and create the gif in photoshop. (although with the beta the gifs are way better than photoshop)
APNG:
