Preheat the oven to 180C/fan 160C/gas 4.Toss the gooseberries with the caster sugar, spread in the bottom of a medium ovenproof baking dish with 2 tbsp water.
To make the crumble, put the flour, butter and a big pinch of salt in a bowl. Rub together with your fingertips until you have a rough breadcrumb texture. Mix in the demerera sugar.
Scatter the crumble topping evenly over the gooseberries. Bake in the oven for 45-50 minutes until top is pale golden and crunchy and the gooseberries bubbling. You can also give the crumble a quick flash under a hot grill to give the top an extra browning if you like.