The gyroball?

I thought it was essentially a hanging slider, like when a pitcher throws a bad slider and instead of siding it just kind of spins up there in the middle of the plate with a bullet-like rotation. This is usually a good pitch to hit but it can be effective if the hitter is looking for a slider and you throw one that has the same spin and everything but never "slides." For example, if a pitcher were to throw it on the outside part of the plate to a right handed hitter, where sliders are typically thrown, the hitter may swing at the spin rather than the ball expecting it to slide away from them but instead it actually stays on the same plane. I imagine for this to be effective you would have to establish your slider as as one of your main pitches throughout the game and then when the time is right you throw a gyro ball up there and the hitter has no idea what the hell just happened.

But then again I have heard multiple interpretations of what it actually is. I have also heard it described as a screwball or a changeup so who knows

It's not a myth, and it's NOT a breaking ball. A gyroball is just a fastball thrown in a different manner...it spins sort of like a football, with the same half of the ball facing the batter the entire time. The Japanese man who apparently invented the pitch created a half black/half white baseball as a training device to throw the pitch...if you throw it correctly, only one of the sides is visible to the batter. This type of spin allegedly creates a fastball that loses less velocity on the way to the catcher, creating more of an 'exploding' finish to the pitch. Dice-K probably messes around with it now and then, but I don't think it's a part of his normal assortment of pitches.

Honestly, I don't know why they call it a Gyroball. It's essentially a screwball, and sure, he throws it. I've seen it many a time.

it is a myth. It gives the batters facing him just something else to think about.

It isn't a myth! He throws it...I've seen it! and ya its Dice-K who throws it