They reduce all the feedback you get from your amp when using pedals.
Agile AL-3000 Cherry Sunburst
Fender American Strat HSS
1981 Yamaha FG-335II
Crate V1512 (USA made)
Fender Frontman 15R
Dunlop Crybaby Slash Wah
MXR EVH Phase 90
Ibanez TS9 w/ Keeley Baked Mod
Boss SD-1
Boss DD-3
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