You can launch files by double-clicking on them in the filelist. All files are stored on your Gmail account so the file must be transferred to your computer before it can be launched.
When you double-click on a file, a local file is created and then launched on your computer. You can only launch one file at the time. While transferring a file to your local computer for launch, you will not be able to launch a second file simultaneously or initiate another copy'n'paste session. If you modify the file and save it, the changes are only transferred back to your Gmail storage when you exit the program you're viewing the file in.
You can also open files from inside regular Windows programs using the Open With... dialog. Currently, opening files from inside the Microsoft Office products may not work with GMail Drive.