Maybe not the "worst" gamble, but one of the least successful. I think Cleveland was aware of the the risk, but they had the best player in the game in the prime of his career, and a young point guard on the step of becoming one of the top players at his position, as well as a lot other young talent. If there's a time to push in a lot of your chips and go for it, that was it. If it works, you get a championship-level team for half a decade.
And it still might work. There was no question there was going to be an adjustment period, and it's not inconceivable that by May this team is humming on all, or at least most, cylinders and goes far in the post-season. They're still in the Eastern Conference and are in no danger of missing the playoffs.
And again, you have to tip your hat to Flip Saunders. He asked for the moon for Kevin Love, and held out until he got it.
I do think the "worst" gamble was made the summer before, and we are the beneficiaries (in the long run). Brookyln traded half a decade of draft picks to add two former superstars who were really more cut out to be role players at that point in their careers to an already expensive team that really was not a championship contender at all, having just lost in the first round after a good-but-not-great season. There was little more they could add to their team beyond those two due to the lack of additional resources and payroll inflexibility created by being $30 million over the luxury tax line, so if anything went wrong in terms of injury or chemistry, they would be stuck. Furthermore, if it somehow all worked, they weren't looking at half a decade of success, but really 1-2 years at most. They paid a similar price as the Cavs, from a lesser position, and with a smaller potential payoff.
(Actually, the worst gamble, or series of gambles, was probably committed by the Cavs in the early 80s, such that the NBA created a rule about trading future draft picks to prevent teams from being too stupid. But in the modern salary cap era, I'll pick the Nets.)