Gumayusi plays so many crucial roles in big matches

He is great at playing the "supportive" role for the team, and I don't think there's any other ADC who can dominate the lane as effectively as Gumayusi does in big matches when it's necessary.
He also creates great clutch plays during teamfights; of course, there are many instances where he gets caught out absurdly, but perhaps because his concentration increases in big matches, the frequency of those mistakes is low.