Max stamina and movement speed for great trolling fun. Have a friend sit on the back with tranq darts and longneck. Run up to people, dart to the head, go "meep meep" and zoom away. Watch the rage in global chat and giggle.
Fighter: If you have a tribe of over 3 people, this is your mount. With the special saddle that the Gallimimus can hold, you can have 2 passengers, plus the driver. With this ability, you can have 2 people fully armed, and they don't even have to worry about moving, now talk about high DPS!
Scout: The Gallimimus is one of the fastest mounts in the game, and this allows for easy scouting and and transport
Incredibly useful tame on Scorched Earth as a taming mount before flyers are avaulable, at leaat for 2-3 player tribes. It's immense speed and stamina mean you can outrun anything while your friends pepper it with tranqs. And when the tranqed animal starts running, you can coast along side it and have your friends release a tranq arrow broadside! Especially useful for large animals (Rex, Paracer)