Using the most recent google oauth plugin with the google storage plugin, I get this output when running a job that's configured to upload the build artifacts to GCS:

ERROR: Build step failed with exception
java.lang.IllegalStateException: An error occurred deducing a username from the provided credentials files.
at com.google.jenkins.plugins.credentials.oauth.GoogleRobotPrivateKeyCredentials.getUsername(GoogleRobotPrivateKeyCredentials.java:130)
at com.google.jenkins.plugins.credentials.oauth.RemotableGoogleCredentials.<init>(RemotableGoogleCredentials.java:54)
at com.google.jenkins.plugins.credentials.oauth.GoogleRobotCredentials.forRemote(GoogleRobotCredentials.java:204)
at com.google.jenkins.plugins.storage.AbstractUpload.performUploads(AbstractUpload.java:413)
at com.google.jenkins.plugins.storage.AbstractUpload.access$000(AbstractUpload.java:101)
at com.google.jenkins.plugins.storage.AbstractUpload$1.call(AbstractUpload.java:367)
at com.google.jenkins.plugins.storage.AbstractUpload$1.call(AbstractUpload.java:364)
at hudson.FilePath.act(FilePath.java:1078)
at com.google.jenkins.plugins.storage.AbstractUpload.initiateUploadsAtWorkspace(AbstractUpload.java:363)
at com.google.jenkins.plugins.storage.AbstractUpload.perform(AbstractUpload.java:182)
at com.google.jenkins.plugins.storage.GoogleCloudStorageUploader.perform(GoogleCloudStorageUploader.java:109)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:779)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:726)
at hudson.model.Build$BuildExecution.post2(Build.java:185)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:671)
at hudson.model.Run.execute(Run.java:1766)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:381)
Caused by: java.security.GeneralSecurityException: An unknown problem occured while retrieving token
at com.google.jenkins.plugins.credentials.oauth.GoogleRobotPrivateKeyCredentials.getGoogleCredential(GoogleRobotPrivateKeyCredentials.java:196)
at com.google.jenkins.plugins.credentials.oauth.GoogleRobotPrivateKeyCredentials.getUsername(GoogleRobotPrivateKeyCredentials.java:118)
... 19 more
Caused by: java.io.FileNotFoundException: (No such file or directory)
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.<init>(FileInputStream.java:138)
at com.google.api.client.googleapis.auth.oauth2.GoogleCredential$Builder.setServiceAccountPrivateKeyFromP12File(GoogleCredential.java:504)
at com.google.jenkins.plugins.credentials.oauth.GoogleRobotPrivateKeyCredentials.getGoogleCredential(GoogleRobotPrivateKeyCredentials.java:188)
... 20 more
Build step 'Google Cloud Storage Uploader' marked build as failure
Finished: FAILURE