Fox Talbot says this is St. John's berry, being ripe about St. John's Day. [This must be John the Baptist, at the end of August, not John the Evangelist, at the beginning of May.] Hence, he says, it is called in Holland Jansbeeren. Jans'-beeren, he continues, has been corrupted into Gansbeeren, and Gans is the German for goose. This is very ingenious, but gorse (furze) offers a simpler derivation. Gorse-berry (the prickly berry) would be like the German stachel-beere (the “prickly berry”), and kraus— beere (the rough gooseberry), from krauen (to scratch). Krausbeere, Gorse-berry, Gooseberry. In Scotland it is called grosser. (See Bear's Garlick.)
Gooseberry fool is a compound made of gooseberries scalded and pounded with cream.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894