My gooseberry bush has been ravaged by gooseberry sawfly. Note how the leaves have been completely eaten away:
Gooseberry sawfly are quite a common pest. Eggs are laid on the underside of leaves, which are then eaten by the larvae as they hatch. When they've eaten enough they burrow into the soil to pupate, after which the cycle begins again. The RHS recommend picking the larvae off by hand but I never seem to see them, or using nematodes which I don't quite have the stomach for (they enter the bodies of the larvae and infect them with a disease). Alternatively there is the option of using insecticides when the larvae first appear.
I may yet need to resort to insecticide, but given that my gooseberry bush is in a pot I thought I'd try another approach first. I decided to remove it from the pot, wash the roots and pot very thoroughly in an effort to get rid of any pupae hiding in there, re-plant the bush and see if that addressed the problem. You can see how thoroughly the roots were washed in the first image below.
I'll post an update next year to let you know whether it's worked!