The information matrix is just the inverse of the covariance matrix.
I recommend you read the page I linked, or just google covariance matrix. Essentially it contains how certain you are in your measurements. (The lower the number the less uncertain you are).
As an example:
The translation matrix between nodes(ignoring rotation for now). Your covariance ...
ZeroSim is built with Unity. It's not necessarily "easy" but arguably easier then Gazebo. Definitely creating and iterating environments in ZeroSim is much easier then Gazebo.
Disclosure: I am a ZeroSim developer.