Hi Josh,
The Google gadget has a settings link, which allows you to tie gadget functionality back to Tab Links within Method. If users have access to the particular Tab Link specified while logged into Method, then they will have access to that functionality in the gadget.
For example, when loading customer information, the gadget checks to see what tab link has been specified for the 'Edit Customer' item on the settings page. If the user can view the Tab Link while logged into Method, then the relevant customer information (including customer balance) is displayed, otherwise the information displayed will be limited (and the balance is hidden).
A similar process flow would occur for the Vendor information. If the user has access to the Tab Link specified for the 'Vendor' item on the settings page, then they will be able to see Vendor information (including the balance).
You can read more about how the gadget works in this Help Center Article.
Concerning your question relating to roles - You have the ability to use 'roles' within Method to group your users if desired. See this Help Center Article for an overview of configuring users. These roles can then be used to grant or revoke access to each particular Tab Link within Method. Tab Links can be edited under the Customize->Tabs->Edit Tab Links screen.
One note for you Josh ... After testing the above implementation, I did find an issue with the gadget. The gadget's Transactions tab is used to display, as you might have guessed, transactions for the current customer or vendor. At the moment, the transactions tab is not filtering based on the user's permissions.
For example, customer invoice transactions will be displayed regardless of whether or not the user has access to invoices in Method. I have created a ticket for this issue in our internal system for our developers to look at. I'll update this post once I hear back.
Jonathan