Gullfoss is an immense waterfall in west-central Iceland that forms the third part of the Golden Circle, alongside Thingvellir and Geysir. It is in a canyon of the river Hvítá, which means "White River". "Gull" means "gold", while "foss" means "waterfall". Interestingly (to me) many of the waterfalls in northern England also have names that include the word "foss" due to Norse placenames.
Incidentally, the tourist centre at Gullfoss has a very nice shop where you can find every tourist item you would have bought in Reykjavik, but didn't, because you knew from reading this node that they were half price.