Good Morning Jon
Glad you also knew about digitrax decoder selector tool on their website.
Sorry I can’t be of more help but FWIW, to me at least I would risk it and order one. Digitrax does say it fits the 2011 LL GP18 and looking also at spookshow photos and digitrax photos of the actual decoder I can’t see any reason it wouldn’t fit. You could give digitrax a call or use their help desk and ask if no one here can assist.
What I did notice was if your loco is post 2008 and pre 2011 digitrax recommends a similar but different decoder and some minor frame mods for which they have posted some instructions as well.
Good luck.
Come on guys surely someone here has actually done this to help advise.