The Gallimimus Saddle is used to ride a Gallimimus after you have tamed it. It can be unlocked at level 29.
This saddle is special as it has 3 seats, meaning up to 3 people can ride on it at the same time. The player sitting in the middle seat controls the Gallimimus, while the other 2 passengers - one facing forward and the other facing backward - can use weapons and other items.
The gallimimus is one of arks fastest tames, and having extra seats means its probably best for group transport and taming.