Gold is a metallic element, number 79 on the periodic table. Though once considered very valuable, by the 23rd century the Federation considered it almost worthless, except for decorative and functional purposes. (TOS: "Catspaw"; TNG: "The Last Outpost", "Time's Arrow"; VOY: "Muse")
The Ferengi valued gold as a precious commodity until the 2370s, after which its use to them consisted of little more than a medium for suspending latinum in. It still remained valuable to some primitive civilizations. (DS9: "Little Green Men", "Who Mourns for Morn?"; ENT: "Acquisition"; TNG: "The Last Outpost", "Captain's Holiday", "Rascals")
Dialogue in TNG: "The Last Outpost" and "The Perfect Mate" suggests that at least as late as 2368, Ferengi considered gold valuable. In "Little Green Men", Quark says to a 20th-century Human that "gold is good". However, by the time of "Who Mourns for Morn?", Quark describes gold as "worthless".
One explanation for this apparent devaluation of gold is the fact that the Ferengi had only recently made First Contact with the Federation. Replicator technology, if introduced to a gold-based economy, could send markets plunging. This may have happened to the Ferengi, who would have experienced a massive gold market crash as a result. Then again, gold may have lost value for some other reason. The comment in "Little Green Men" may be in the context of a pre-replicator economy.