You won't get the 'set' without sugar. Lemon juice is sometimes added but as gooseberries are already acidic, this won't help. You could use a product called Certo, made from pressed apples that are rich in pectin, in conjunction with some grape juice, they have a recipe on their site for a sugarless plum jam - you could try and adjust. One thing, only make a small amount, without sugar once opened it has to be kept in the fridge and eaten up, it goes mouldy quickly.
http://www.certo.co.uk/Recipes_Framed.htm