Is there a tool

I'd suggest avoiding sandpaper unless you have to. Depending on how hard the glue/gunk is, you might end up removing too much wood and then the dowel wouldn't fit as well in the new joint. If that happens, glue with epoxy, maybe adding microballoons.

If you can soften and remove the glue without removing wood, you will likely have a stronger final joint.

If I couldn't soften the glue, then I'd try a card scraper before I'd use sandpaper, again trying to not remove too much wood.

cific tool for this.

Dowel chucked up in drill/drill press, strip of sanpaper looped over dowel sanding until diameter is purr fect.

I would use a utility blade pocket knife and perhaps a 1/2 inch chisel for places the knife could not reach. Good practice to control these tools on a job like that.