The discovery of gold was accidental, and even those who did realize that it was gold and that there was likely more gold nearby, really had no idea of the extraordinary amount of wealth that later would be panned out of the rivers. They didn't understand what they had, and at first most were skeptical that it wasn't just a fluke lucky discovery of a few bits of gold.

The news of gold in California was initially talked up and popularized by Sam Brannan who stood to make money by attracting people to his general store that he had just opened at Sutter's Fort near where the gold was discovered.

Fatefully, Brannan . . . guessed that as much money might be made from the miners as from the mines, and he opted for publicity. He purchased enough gold dust to fill a jar and traveled to San Francisco. . . . He paraded conspicuously about the town, waving his hat and shouting, "Gold! Gold! Gold from the American River!" The rumors had preceded him, but this material evidence made the rumors suddenly credible. . . . The small population of San Francisco nearly vanished overnight in the direction of Sutter's mill. (H. W. Brands, The Age of Gold: The California Gold Rush and the New American Dream, New York: Anchor Books, 2002, p. 43.)

But gold was first discovered 4 months before Brannan ever went to San Francisco. Apparently the initial gold discoverers, James Marshall and John Sutter, were more concerned with getting their sawmill built than gold. They understood that they had found gold, but since laborers were so scarce, they tried to keep the news as private as possible because they were worried about losing their labor force to gold rumors. Keeping the discovery a complete secret ultimately proved impossible. (Apparently Sutter himself like to talk and he had a cook at Sutter's Fort who couldn't keep a secret either.)

Rumors, of course, spread, and some went to look for gold, but most Californians were initially skeptical that there was any truth in it.

Even then the military governor was assured by a visitor to Monterey that the reports from the American River were untrue. "A few fools have hurried to the place, but you may be sure there is nothing in it." And the editor of one of the two little newspapers published at San Francisco denounced the whole business as "Superlatively silly," "all sham, a supurb [sic] takein as was ever got up to guzzle the gullible." (Rodman Wilson Paul, Mining Frontiers of the Far West, 1848-1880, Albuquerque: University of New Mexico Press, 1963, rev. ed. 2001, p. 13.)

But by late May 1848, spurred by Brannan's advertising with gold dust in hand, the entire towns were reportedly stripped of men as they all flocked to the American River to search for gold. And it just snowballed from there.

TL;DR: Gold was accidentally discovered, and no one understood the magnitude of what they had found. The discovery of gold was mostly just rumor for 4 months until Sam Brannan advertised the discovery with proof in order to drum up business for his general store at Sutter's Fort.