One of the cards literally gives you pills that make you one year younger for every pill while you retain all the knowledge you had. The card gives you 100 pills. That is a potential reward for winning the game. Meanwhile when Gon uses Nen to age himself forward it was almost treated like it was the craziest shit in the world. Yet Greed Island it is a simple feat among all the other crazy shit these cards can potentially do.
There's quite a difference between your body de-aging to a previous physical state, and what Gon did. Gon not only aged up, he obtained his future potential in that moment, ie all the training he should have had to do, in exchange for basically killing himself.