Global here refers to the geographical spread of the service, GKE in this case, measured in regions, not the number of services.
Edit: I saw your point a bit late. It was limited to GKE, which makes my initial comment about "service" incorrect, and it was global, which keeps my comment about "region" correct. On a related note, an SRE from GKE posted on Slack that GCE was out of resources and so GKE faced resource exhaustion as well [1][2] - so it _might_ have been a multi-service outage.