what do they actualy do?
They reduce all the feedback you get from your amp when using pedals.
would it kinda clean up sweep picking any?
Maybe a tiny bit. What a noise gate (or suppressor) does is sets a threshold for all the sounds to come through. If the volume of the noise is above the threshold, it sounds. If the volume is below the threshold, the noise is taken out. Useful, especially if you use high gain.
It would only clean up the buzz you get from your pickups. So no, it won't clean up your sweep picking, it's all in the technique
cool, thanks for the info.