Gristlebeak is an NPC that you are going to have to fight as you quest your way way through Shadowlands. While you may be able to spot her flying around to the right of Theater of Pain, you may have difficulty actually finding out how to get her to fight you.
In order to summon her for a fight, you are going to have to destroy her eggs.
You can find these in Maldraxxus, at the following coordinates: 7.55, 51.51. You will see the four large eggs there, with Gristlebeak patrolling the area in the sky. Simply attack the eggs to destroy them.
Once all four of the eggs are destroyed, Gristlebeak will come down to see what you’ve done. You’ll see her health bar turn red, meaning she is now aggro and is ready to be fought.
The fight it fairly straightforward and Gristlebeak doesn’t have any mechanics you need to worry about watching out for. If you’re leveled up enough, you will take her down in no time.