If you access Zoho Invoice through your Google apps account, then the contextual (Zoho Invoice) gadget will be displayed automatically and this cannot be deleted. However, if you do not wish to see the gadget in your Gmail account, then you can disable this app for the user. Once this app is disabled for the user, they will not be able to access their Zoho Invoice account under the universal navigation bar 'More'.
Henceforth, to access the Zoho Invoice account you will 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 Google Apps Developer Blog.
Zoho Invoice FAQ