Gantter for Google relies on Google’s Share API that is occasionally not available from Google's end. 

However, you can also share your projects directly from Google Drive:My Drive. 

You can also try clearing your browser's cache or use Chrome's incognito mode (press Ctrl + Shift + N to bring up a new incognito window) and then try again from the Gantter editor. 

Your Gantter account must be signed into your matching Google account to share a file. If the sharing option worked in an incognito window this tells us that your Google and Gantter accounts are not matching in the browser being used.