According to this issue, the maven-gae-plugin released an update and it took several hours before this was available from Maven Central.
Who runs Maven Central and what causes this delay?
Maven central is handled by maven developers (Apache).
And it takes time to check if the new version of an artifact respects some conventions given by apache.
As said on maven.apache.org
The [..] process is mostly automated with turnaround time averaging 1 Business day.
Resources :