Whenever you start composing a message on Gaggle Mail you can save a draft copy so you can come back to it later. To save a draft just click the Save Draft button at the bottom right of the compose window.
Once saved, any drafts you have will be listed at the top of the Messages list for that group. Only you can see the drafts you've created. Just click on a draft to open it up and continue editing or click the trash can if you want to delete it.