Yes it's possible.
1. Browse to the folder where the templates are saved. The default location is Documents/Google Web Designer/templates/
2. Open the html file in a text editor.
3. Search for gwd-template-data and you can set the "label" to the same name for multiple templates. When multiple templates have the same label, they will show in a group or category and the size dropdown will be the dimensions specified in the gwd-template-data.
4. Save the file.
5. Relaunch Google Web Designer.
Hope it helps.
San
Google Web Designer team.