I would not use gelatin for horn wood joint... but for sinew layer it can be used ...
Books with Masters of hornbows recommend ONLY fish/sinew (mix) glue for horn-wood glue joint and PROPER sizing (both parts) with thin solution of fish/sinew glue, at least (!!!) 20 layers of thin hot glue.
GENTLY Heat both parts (wood, horn) with heat gun before each sizing and wait to dry each layer !!!
Whole procedure of sizing took me couple of hours.
Then add additional fish/sinew/hide glue parts into melting pot and make THICK solution.
Then just one very thick layer of HOT fish/sinew/hide glue (I mixed all because run out of home made cat fish glue) and quickly press together and do fixing with rope-tencik or clamps.
After fixing GENTLY heat up all with heat gun and let dry one day ...