Now I see the gap you refer to. It is hard to see in the photos so I missed it. New roots may be reluctant to grow where there is already a strong root growing - hormone or no but it can't hurt to try. I suspect you'll need to cut it off to get new roots to grow strong enough to replace it.
If it does not show up in a photo and if you can make that side the back so it is not obvious maybe it doesn't matter so much?
These roots worry me far more. They are the ones I would concentrate on getting close to the rock
callistemon roots.PNG
Small ones marked in red can be removed completely. Black area needs to be closer to the rock. If possible, hugging the rock surface.
Earlier I mentioned roots growing under or over each other.
Callistemon roots 2.jpg
The root marked blue here will always be pushed away from the rock as the ones under it thicken.

you need to remove either that one or the ones under it. Choice will depend whether you want bendy roots or straight down the rock. Personally I'd take the straight one off and keep the ones showing movement. Also look for other places where roots cross to see if some more need to be moved or removed.
Roots will fuse just like trunks and can look very natural if it happens right but you will need to keep the pressure on them to stop them lifting before they fuse.
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