I saw your question because I was having exactly the same problem and I also did the same steps as you.
The solution to give user root full control of the mounted cloud folder :
You have to go to your Google Cloude place, search for "Service account" and clic on it.
Then you have to export the key file of your service account (.json)
(I have created a new service account with the Google Cloud Shell consol using this command : gcloud auth application-default login
And then followed the steps when I was prompted by the shell.)
Clic on Create Key and choose JSON
Upload the .JSON keyfile to your linux server.
Then on your Linux server, run this command : gcsfuse -o allow_other --gid 0 --uid 0 --file-mode 777 --dir-mode 777 --key-file /path_to_your_keyFile_that_you_just_uploaded.json nameOfYourBucket /path/to/mount
To find your root user GID & UID, login in root to your server and in terminal type : id -u root
for UID & id -g root
for GID
Hope I helped, because I've been struggling for long and no resource on internet really helped. Cheers.