The purpose for which you intend to use your gooseberries determines when you should pick them. Gooseberries taste different during different stages of ripening, and therefore can be used for different taste applications. For example, green gooseberries, which are just ripening but not fully ripe, are more bitter, and therefore are often used in jams, sauces, pies and pastries, where they are mixed in with other flavors. Gooseberries intended for straight-from-the-bush fresh consumption aren't harvested until they are fully ripe.
Pick early gooseberries for applications like jam when they are green, yet full-sized. The time it actually takes for the gooseberries to ripen to this stage varies depending on when you planted and your climate conditions; spring sprouting gooseberries are often ready for picking at this stage in early to mid summer. When you are picking these fruits, they will be hard so you don't have to worry about squeezing too hard. Pick every other berry to allow the remaining berries plenty of air and light to continue to ripen.
Gooseberries will be fully ripe by the end of summer. You know they are ready for fully-ripened picking when they are red or pink and feel squishy and soft to the touch; these are the berries that can be eaten directly from the plant for the sweet, fruity flavor. Be very careful harvesting the rest of the berries, since they are extremely soft and will easily squish open if not handled gently.
When picking gooseberries at any time of year, be careful of the plant's thorns, designed to keep you from picking. Wear garden gloves and long sleeves to protect yourself. These thorns will not keep birds away, so consider covering the ripening plants with bird netting to make sure you get some of the fruit as well. After harvest, use either partially-ripe or fully-ripe gooseberries within four to five days. They can last a little longer than that in sealed and refrigerated containers, but these berries will not be healthy for long when removed from the plant.