Well basically he was hired by Blizzard when WoW was at its best (on the way to 12 mill subs). It only took him a short amount of time to make the game go down a spiral. He joined some months before the launch off Wrath of the Lich King (arguably the most epic expansion ever made for the game) and thankfully didn't have much to do with that expansion, but you could already see at the very last patch before the new expansion launch (Cataclysm) that he had plans to casualize and simplify the game (he merged stats because they were "too complicated" and introduced raid finder which basically dumbs down every single raid into kindergarden-level difficulty).
The expansion came and the sub numbers instantly tanked by several million and it remained that way until the next expansion (Mist of Pandaria) of which he was also responsable. Here the sub numbers kept tanking even harder and at the end of the expansion numbers were at below 6 million. He quit /got fired (idk) mid-expansion because he saw where it was going. Another expansion launched and it was PURE garbage, then a new one launched last year which was a hugeee improvement over any of Ghostcrawler's works.
Basically what he did was casualize the game through simplification, destroyed PvP balance as a result (WoW was actually getting quite popular in the PvP scene, he made it unplayable and non-competitive), he made several unpopular decicisions without listening to the community and so on.
Basically he failed at his job and Riot decided that must mean that he's qualified to develop League. The current state of the game can't exactly be thought of as great tbh, and I believe GC is largely to blame. The guy has no sense of what balance actually means.
and decided to give himself a new skin.
