Currently in the new Google Drive you are not able to add a document directly to the template gallery. By going directly to the template gallery at https://drive.google.com/templates you are able to add your templates there.
Submit a Template
Note that currently the new Google Sheets does not upload to the template gallery. There is also a lag between when you upload and when they appear in the gallery.
Students will link to the URL and then click on “Use this template.”
While previewing one of your documents notice that your name is hyperlinked next to the template title. Click on your name to filter templates by you. Copy the URL while looking at your list of templates. Sharing this URL will take students to all of your templates. Click Here to see my template gallery.
If you are using Google Classroom you will probably not need to use the template gallery. However, there are many collaboration reasons outside of the classroom that might necessitate the gallery.