Grails is experiencing an outage with their artifactory service that requires using a non-standard URL. Unfortunately, the standard URL is hardcoded into the grails wrapper. See apache/grails-core#11825 (comment)
To get the target project to build:
cd $grails_wrapper_directory>;
./gradlew clean assemble;
cp wrapper/build/libs/grails4_1-wrapper-3.0.0.BUILD-SNAPSHOT.jar $target_directory/grails-wrapper.jar;
The grails wrapper contains a layer of indirection in starter/build/libs/grails-wrapper.jar.
We don't use this, because this itself just pulls down the latest wrapper/build artifact from a remote.