Heat oven to 180C/160C fan/gas 4. Butter and line a 20 x 30cm traybake tin with baking parchment.
Put the butter, caster sugar, flour, eggs and orange zest in a bowl. Beat thoroughly with an electric whisk until creamy and smooth. Stir in the gooseberries, then spoon into the tin and level the surface. Bake for 35 mins until a skewer inserted into the cake comes out clean.
Stir the orange juice and granulated sugar together, spoon over the surface of the warm cake and leave to cool and set. Cut into squares.