Good day @Orlandoc6,
You have to install git, it's one of the first steps in setting up your environment as seen in our docs. You'll have to follow the full procedure if you want to simulate with gazebo.
If you are new to this kind of thing I'd recommend taking your time looking at the docs as there's a lot of steps to take. As mentioned in the docs, we recommend working with Ubuntu 14.04 64 bits. If you have problems setting up your ROS installation or workspace I'd look at their wiki and/or their forum.