Geroge Gillett is a very successful (American) businessman, a chairman and director of several companies in which the Gillett family holds interests including the Montreal Canadiens, arguably the most successful NHL team of all-time, and he is a member of the Board of Governors of the NHL.
As for whether or not he will be good for the team, some owners prefer to oversee the management of team affairs (Mark Cuban, Jerry Jones, Dan Snyder, Harold Ballard, Al Davis, etc.) while others understand that their strengths lie as businessmen and not as the team's General Manager. I see Mr. Gillett as the latter, not the former. A good thing, in my opinion.
As owner of the Montreal Canadiens he has taken a hands off approach. He has let his general manager hire smart, intelligent, people who know the sport. He plays no favorites. He's the kind of owner you want, although he hasn't always been quick to cough up the cash for a good player. But if your team is close to winning the championship, he is willing to shell out if the right guy can put you over the top.
I'd look to Gillette to be a mid-level team owner in the Premiership...he won't spend extravagant cash like Chelsea but he may pick up a key guy near the end.