Ok so to tame one you need to get something that can fly and pick it up.

Step 1. Put a 4x4 hole in your roof.

Step 2. Find a gallimimus on a bird.

Step 3. Pick it up.

Step 4. Put it in the hole you made.

Step 5. Get out a bat or tranq arrows and knock it out.

Step 6. Feed it berries or crops and get a few narcotics.

Step 7. One your gallimimus has woken up and has been tamed, get your bird to pick up your new friend and put it outside (If you want you could leave it in your house.)

Congratulations! You have tamed a gallimimus!!! :D