Cloud functions can use go modules or vendor. The vendor method is preferred because private repos are easily packaged.
If you have intended to use vendor but use go mod, you MUST .gcloudignore the go.mod & go.sum files.
The .cloudignore should stay in the same directory from where you are invoking gcloud functions deploy.
$ cat .gcloudignore
# This file specifies files that are *not*
# uploaded to Google Cloud Platform using gcloud.
# For more information, run:
# $ gcloud topic gcloudignore
#
go.sum
go.mod
.gcloudignore
