No, not really. The one requirement for cooking with the sun is (yep, you guessed it) SUNLIGHT! If the sun is sitting behind clouds all day long, you won't have much opportunity to cook with GoSun. You might be able to heat up some leftover soup, but that's about it.
However, if theres occasional cloud cover on a mostly sunny day - you should be good to go. If while cooking a cloud or two pass over, the GoSun will remain hot during times of quick cloud coverage, and the food contents will continue to cook. Because the cooking tube acts as a great insulator, a few occasional clouds (on an otherwise sunny day) will not interrupt your overall cooking experience.