Put half the gooseberries and 1⁄2 cup water in a large, heavy saucepan and bring to a boil over medium heat. Crush berries slightly with a potato masher, add remaining gooseberries, then add sugar. Mix well. Bring to a boil, reduce heat to medium-low, and simmer, stirring often, until berries have softened and broken down, about 10 minutes.
Now start testing: Spoon a little of the gooseberry syrup onto a saucer that's been chilled in the freezer. When you tip the saucer, the mixture should be thick and jammy, not runny. If mixture is not thick enough, continue boiling and testing until thick. This may take an additional 20 to 30 minutes, as the pectin content of the berries will differ with variety, size, and season.
Allow jam to cool, then transfer to containers, cover, and refrigerate until ready to use.