Has GT ever had paid dlc? I never looked at it but I assume so as they had paid demo's.
Nope. GT6 had the ability to buy credits with real money if you just want quick access to buying certain cars, but that's it.
Game had lots of patches and some free content updates with them.
(Edit: I guess maybe GT5 had them as per a prior post? I only played GT6 where everything was free)