Mildew on gooseberries - some varieties are more resistant than others, I grow Invicta, which has never suffered (fingers crossed here!)
Increased air circulation helps, this is one of the reasons pruning to open out the centre of the bush is done. It's worse on lush growth, so don't overfeed the bushes.
You can spray with a fungicide containing copper such as Bordeaux mixture, or try spraying with a solution of bicarbonate of soda in water, or you could use Multirose from PBI which contains myclobutanil.