how is bork on jax

Gunblade > botrk on jax

Depends on a lot of things.

Depends if the ennemy team is "balanced", heavy AD, heavy AP, tanky or squishy.

Against squishies and/or heavy AP, Gunblade is by far better because it gives a bit of AP (boosting your R active bonus MR) and better burst to kill them quickly. Against mobile ennemies and / or tanky ennemies, BORK is better because it gives you extra MS on top of slowing your ennemies through the active and it gives better DPS to bring down the tanky ennemies. BORK is also better for splitpushing thanks to the extra AS.

Both of them are very good on Jax but I found that they don't have the same role, depending on the game.

gunblade is better to jump on squishies and burst them.

botrk was heavily nerfed. and it isn't worth it anymore.

This thing is good only when the enemy has at least 3 tanks, and it's only worth it on on-hit champs if comined with and even then it's still outclasse by every other item in the game... it's a noobtrap right now.