Gantter for Google relies on Google’s Share API that is sometimes not available from Google.
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 in to 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.