This is the definitive answer for nuvi models:

Quote Originally Posted by catymag View Post
New gimgunlock-0.03+MapSigPatch - Windows version Latest release
As usual drag&drop the img in it to unlock it.
In post #256
It unlocks NT (not NTU) IMGs in newer devices, but not working with all new devices 2xx9,
If you have 2xx7 (8) (PartNumber> 006-B1582-00), works
If you have 2xx7 (8) (PartNumber> 006-B2077-00), doesn't work.
From the initial post of [Only registered and activated users can see links. ].