Holonomic + gyro/PID

[Jogo]

I've been doing some research on holonomic drivetrains, and understand the physics behind it. I'm assuming that, of course, error will begin to mount up, which requires some additional programming to overcome.

I've heard people say they use a gyro and a PID loop, but I'm confused as to how the gyro is implemented. And, even if the gyro is used to control the angle, how can you control the velocity? Doesn't the gyro itself rack up some error over time?

I'd appreciate any advice and any good links. Thanks!