Perhaps you've played the game of Telephone as a kid, that's where one kid tells a story to someone, then that person repeats the story to someone else, and so on. By about the 6th or7th person the story is all jumbled up.
That's what happened in the news of the 1848 gold discovery in California. It took many months to travel out to the world, and about a year later the miners (and everyone really) were flocking to California. As the word of gold traveled across the Pacific to China the story simply became - Gold Mountain. The belief was there were mountains of gold in California.
So if you were a young man (or an old man) in Guangdong province (a coastal province) in China, you would simply tell people "I'm going to gold mountain." Which would be like saying, "I'm going to California to mine gold."
Of course when Chinese immigrants arrived they found a very different reality. And that is where the 'Gold Mountain' comes from in the title. So in reality any mountain in California could be ... Gold Mountain.