There may be two database files in your configuration directory, access_tokens.db and credentials.db. I would try renaming them one at a time, and letting gcloud recreate them. On my Linux machine I would be running the following:
# Display the location of the database files
gcloud_config_dir=$(gcloud info --format="value(config.paths.global_config_dir)")
echo $gcloud_config_dir
# If this didn't work, try using $HOME/.config/gcloud
# Rename the credentials database and test gcloud again
mv ${gcloud_config_dir?}/credentials.db{,.old}
gcloud compute instances list --limit=1
# If necessary, rename the access tokens database and test gcloud again
mv ${gcloud_config_dir?}/access_tokens.db{,.old}
gcloud compute instances list --limit=1