GMail gadgets for Zoho Invoice will be automatically enabled for the users who installed Zoho Invoice from Google Apps Marketplace.
However, if you do not wish to see the gadget in (footer) your Gmail account, then you can disable the app for the user. Once the app is disabled for the user, the users will not be able to access the Zoho Invoice account under the universal navigation bar 'More'. Please refer the image below:
To access Zoho Invoice account, the users need to get into the login page (https://invoice.zoho.com) of Zoho Invoice and sign in through the Gapps account. For more information on how to disable the app, refer the
Zoho Invoice FAQ