They released tons of free content updates over the years. For me, it got stale pretty quickly because you needed increasingly huge piles of in-game money to participate in any of the new content and that means either grinding for hours or buying fake money with real money.
The single player and the world building were top notch though and it still doesn't feel like an old game after 10 years. It's crazy to think GTA 5 is now older than San Andreas was when 5 was released.