Gaia is deeply integrated into the editor environment and because of this Gaia needs your project to be kept in good working order.
If your Gaia manager does not display then it will be because of a compilation error. To resolve this open your Console window and then hit the clear button. Any compilation errors will remain and show up in red. You will need to either fix the errors, or remove the asset or script that is creating them.
When you have done this allow Unity a few moments to recompile, and Gaia manager will be available for use.