How to increase root disk of Google Compute Engine ?

 Resize the persistent disk in the Google Cloud Platform Console:

1. Go to the Compute Engine page.
2. Click on "Disks" in the sidebar
3. Click the name of the disk that you want to resize.
4. At the top of the disk details page, click "Edit".
5. In the "Size" field, enter the new size for your disk.
6. At the bottom of the disk details page, click "Save" to apply your changes to the disk.
7. After you resize the disk, you must resize the disk partitions so that the operating system can access the additional space.

 Then, on Debian/Ubuntu/etc. run:

$ sudo apt install -y cloud-utils         # Debian jessie
$ sudo apt install -y cloud-guest-utils   # Debian stretch, Ubuntu
$ sudo growpart /dev/sda 1
$ sudo resize2fs /dev/sda1

or, for RedHat/Fedora/CentOS/etc.:

$ sudo dnf install -y cloud-utils-growpart
$ sudo growpart /dev/sda 1
$ sudo xfs_growfs -d / 

Refer from https://stackoverflow.com/questions/22381686/how-can-size-of-the-root-disk-in-google-compute-engine-be-increased