I tried taking snapshot of disk attached to VM but there was no option to specify specific GCS bucket location. There is only an option for selecting multi-regional or regional location where the snapshot will be stored.
Directly exporting a snapshot is not supported.
You can create a disk from the snapshot, next create an image from the disk. You can then export the image to Cloud Storage.
Note: You cannot export Windows images. You cannot export Google created images. Only non-Windows images that you created.