doc: Add line to choose GCP zone freely
@@ -19,7 +19,7 @@ If the `gcloud` command is not installed on your PC, please refer to [this docum
### Create GCP instance
-Create a GCP instance for Rancher Server.
+Create a GCP instance for Rancher Server. This document uses the `asia-northeast1-c` zone, but you can choose any other zone you want.
```console
ZONE=asia-northeast1-c
