In order for Gaia to work properly the first time it must be compiled by Unity.

However, if your project has errors, this can not happen, and Gaia won't show up in your window.

The way you can see what is causing the issues is to looking for lines that are colored in RED in your console window.

You need to either fix the compilation issue or remove the offending asset.

After you have gotten rid of all the compilation issues, Gaia will then compile, and you can start using it.