They do provide an effective way of preventing ESD because they insulate the devices you're touching from your charged fingers. Since the ESD normally won't be strong enough to arc around or through the rubber, they're generally effective.

Are ESD gloves enough?

ESD gloves should be considered as mandatory safety equipment for working in most laboratory, cleanroom, biotec, and electronic works. However, ESD gloves must be used in conjunction with a grounding wrist strap in order to ensure the absolute lowest possibility of electrostatic discharge.

Do gloves stop static shock?

They will not help nearly as much as a real ESD kit. Rubber gloves are so thin, even the 6mil ones, that and significant static discharge will go right through them.