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 ...
I've made all of my bows with hide glue. Some of those have self made sinew glue on the sinew backing, but they are the worst glue jobs... Making your own glue is not always so easy. We need to remember, horn-wood glueline is the ultimate test for your glue, stresses in sinew backing are nothing close to it. It mostly boils to that its so hard to get the glue stick in to the horn part.
Another thing about the glueing and sizing is, when laying the first layer of hide/sinew/gelatin glue on the horn or wood core, you need to heat up the pieces and use thin glue. This will make the surface suck up more glue into itself, cause with the extra heat glue will stay longer liquid and run down to those fibres. After this heating is not recommended, since it degrades the glue, making it more brittle and we dont want that. After laying out the layers, what ever amount it is (i've used with success 5 to 60 layers in total) you heat up the pieces gently and use really thick glue for the last layer. Please don't use heat gun, you can really fast destroy a lot of work, hot plate or oven in low temp will do. With a lot of work i mean, its best to lay out the layers over some time so the glue hardens between the layers, maybe in 3 hour intervals. Also i allways let the last layer cure for 24 hours.
Finally when gluin the parts together you want to use thick glue, so it will fill all the small cracks and miss match in the core, but also you want to have it not setting until the clamps are fixed on place - this is why we heat up the core and horn, again to keep the glue running. So you will have only two times when you heat up the core and horn. Adam Karpowicz goes to detail in his book about Turkish bows, so if you are interested, you really need that book.
In general you just want decent hot glue in every possible place and it staying in its place. Gelatin/gnox/hide glue will definitely do. If someone wants to go really deep in to hornbows, then maybe a isinglass glue will be a good option but i would prefer to first practice with something else than buying some glue that costs 50-100 euros for 1 bow.
Mafort: All these tips work for a wood-horn-sinew bows or horn-sinew bows. Just go for it!
Now looking back, the guy only asked do you need to have your glue hot and we started to ramble!
Yes, have the glue hot! Hah.