I noticed that there are no completions for fish bundled with google-cloud-sdk, I inspected the bash/zsh completions which are python argcomplete.
Fortunately fish already has a function bundled to support this, so all you have to do is the following:
complete -c gcloud -f -a '(__fish_argcomplete_complete gcloud)'
complete -c gsutil -f -a '(__fish_argcomplete_complete gsutil)'
Would be nice if this was bundled with google-cloud-sdk so it can be utilized by all the dists.