This add-in isn't supported anymore and, it appears the project has been abandoned. If you visit the official website, a message will greet you saying the following:
Thanks to all our customers for 12 great years! We are no longer selling or supporting the Getting Things Done Outlook Add-In. For more information, read the letter from David Allen and NetCentrics. Please check the Knowledge Base as it has been updated with articles dealing with the most common customer support problems, including activation issues.
As you can see, the product has been discontinued, however, there is no information for Outlook 2016 and Windows 10.
For additional info, consult the official forums: https://www.gtdoa.com/forum/viewforum.php?f=10