You can use the "Visualization" feature to see reports based on multiple projects at a time. Please use the following steps:

1. Go to the Gantter home page. You can use the following link for Gantter for Google Drive: https://google.gantter.com/gad/drive/glp

2. Select more than one project on the home page, the button at the bottom-right of the screen will change to show the visualization option.

3. Click on the button at the bottom-right of the screen

4. Once the visualization have loaded, you will be able to use the tabs to see different reports based on the projects you had selected.