While that may be true what your not realizing is that both Mario and Pokémon have been around far longer in the market than GTA was.
Very first GTA was released on 1997.
Mario as a character made his first appearance in 1981 in a Donkey Kong Nintendo Arcade Game.
Pokémon was released in Japan in 1996 and it also has far many more games than GTA does. It also sold 440 million copies while GTA is at 380 million.
GTA doesn't have shirts or any merchandise, or any kind of films or movies either. They only have 5 mainline titles with a few spin offs here and there that are much smaller.
I am pretty sure if GTA6 was announced today alongside Mario the thread for GTA6 would.be far bigger and more active than a new Mario title announcement. GTA simply has more worldwide appeal.