The system requirements for Extender are the same as those for Microsoft Dynamics GP. You can review the Microsoft Dynamics GP 2016 System Requirements documentation on CustomerSource or PartnerSource.
Extender will work with the Microsoft Dynamics GP Web Client. The Extender Menus and Forms do not show in the web client, however, due to the way the GP Web Client Organizes the menus. In Extender, you can add a menu to the Transaction, Cards, Inquiry, etc top level menus. On the GP Desktop client, the Extender menus and therefor the Extender form will be visible. If a user turns off the menus in the GP Desktop client and just uses the Navigation Pages, the Extender menus/forms will not appear. This is the same in the GP Web Client as it does not use the menus and only has the area pages. The workaround to this in the GP Web Client is to use the Home Page Quick Links. One of the options in the Quick Links is "Extender" and this will allow you to access the menu/form that was created in Extender. Once an Extender form is added to the Quick Links, it will be available in the desktop client and the web client.