To access Gaia select Window -> Procedural Worlds -> Gaia -> Show Gaia Manager:

If the Gaia menu is missing then this means that you have a problem with your project which is stopping Gaia from compiling.

This is no fault of Gaia's as Gaia does not contain compilation bugs - it is an issue with your project. You will also find that Unity will not play your project while compilation errors exist.

Open your Console window by selecting Window -> General -> Console:

Take a look at what is causing your issue by looking for lines that are colored in RED in your console window.

You will need to either fix the compilation issue or remove the offending asset from your project.

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