Hi @MyBella, welcome to the community!
We want all the world’s music on Spotify. However, some artists and tracks are not currently available. Sometimes agreements can’t be reached with the artist or label, or a change may happen in music ownership.
Thousands of albums and artists are added to Spotify every day. If you can’t find an artist you’re looking for, they may appear (or reappear) soon.
In the meantime, feel free to contact Garth Brooks or his label to ask when he plans to be on Spotify.
Let me know if you have any other questions!