AD Authentication is not supported at all on GAIA. A TACACS+ user does not have to be created on GAIA though. All you need to do is setup the TACACS connection:
add aaa tacacs-servers priority 1 server 1.2.1.3 key ***** timeout 5
set aaa tacacs-servers state on
set aaa tacacs-servers user-uid 0
set aaa radius-servers super-user-uid 96
add rba role TACP-0 domain-type System all-features
That should suffice, how the TACACS server itself will handle the request for the user that is not there in the user list, I don't know, we are also struggling with that part of the implementation.
Regards, Maarten