Hayward has played very well this year, but I agree - to an extent - that he is more of a luxury than a necessity. If the team had, say, Joe Ingles, instead of Gordon Hayward, I don't think our team would suffer all that much. As VG pointed out, maybe having a true PF or swing would help, but Gordon's talent more than makes up for who we would have instead. And with Tatum and Gordon out there, along with Brown, we actually have a very formidable wing rotation with our overall size.
KG might also be alluding to chemistry. If you recall, Kemba, Tatum, Brown, and Smart were all part of Team USA, while Hayward was not. I don't think KG is correct - if we have a healthy Hayward for the entire playoffs, then the Finals would be a slam dunk - but I guess I understand what he is saying.