Depends where you live and which variety you have, you'll know they are ripe when the gooseberry goes slightly soft.

Depends whether you want to eat them raw or cook them. If you have a heavy crop you can thin them out, taking one from each pair (they often grow in twos) and this will allow the remaining ones to grow bigger for dessert use.
The thinnings are good stewed with sugar, used in crumbles or pies (great with a few strawberries added) or make a sauce to serve with mackerel ( which we did at a beach barbie with some newly caught ones last week - yummy) I start eating odd ones asap, even when they are so sour they make my teeth go funny. So basically, pick them as soon as you like and make sure the birds don't get there first.