It will be American Gooseberry Mildew (or blight) and can be a real pain. There are some varieties now that are immune to it. A severe attack can strip all the leaves from the bush but the bush recovers the following year. Try to strip off some of the new growth to let more air into the bush. Spray with bordeaux mixture. You can rub the mildew off affected berries if you have the patience - it doesn't penetrate below the skin.

Sadly once you have had it it will be back another year so you might wish to invest in a resistant variety - Invicta and Greenfinch are recommended and I've had no blight since growing them.