I think they still charge by hour/min, but unlike your instance, they are charging you as long as you keep the disk space. Like I said, you can only INCREASE the disk size but not decrease it dynamically, so I suggest start from smaller disk space and only add more when you need it.

https://cloud.google.com/compute/docs/disks/add-persistent-disk

There are no easy way to “decrease” disk space, the alternative is creating a smaller disk space, copy your data to the new smaller disk, then attach your instance to the new disk and delete the old one.