If you have a Compute Engine custom image that has a bootable operating system on it but does not have the necessary Compute Engine drivers or guest environment packages, you can use the image import tool to make that image bootable on Compute Engine.
Use the --source-image flag to specify a custom image to make bootable, instead of using the --source-file flag that specifies a new disk to import.
gcloud compute images import [IMAGE_NAME] \
--source-image [SOURCE_IMAGE] \
[IMAGE_NAME] is the name of your destination image.
[SOURCE_IMAGE] is the name of your source image.
[OS] is the operating system of the SOURCE_IMAGE. It must be one of the following: centos-6, centos-7, debian-8, debian-9, rhel-6, rhel-7, ubuntu-1404, ubuntu-1604, windows-2008r2, windows-2012, windows-2012r2, windows-2016, windows-2019 or a BYOL-licensed image.