You copy files using the normal Windows Explorer tricks.
To copy files; select the files in the list. Then choose Copy from the right-click menu. To paste the files, navigate to the destination folder and choose the Paste menu-item.
You can also just drag the files and drop them on the destination folder.
By default GMail Drive copies files during drag'n'drop operations, but you can control the effect by using the SHIFT and CTRL keys when dragging.
You can drag files to any Windows Explorer folder - even some 3rd party programs accept files being dragged onto their window.