Dennis Gartman told CNBC's "Futures Now" on Thursday that his gold call last month couldn't have been more off the mark.

Toward the end of August, Gartman had made the case that gold would outperform .

"I think the stock market has a little bit further to go on the downside, perhaps another 25 or 30 S&P points," Gartman said on the Aug. 22 episode of "Futures Now." "So if I had 'X' amount of money to put to work, I'd put it to work in the gold market."

What's happened since then? To put it bluntly, that turned out to be dead wrong.

From Aug. 22 to Sept. 12, the S&P added 30 points, while gold slid $40. Overall, the metal underperformed the S&P by about 5 percent.

"Clearly I'm wrong on the gold, and there's no reason to be anything other than truthful about it," Gartman said.

(Read more: 'Pre-emptive selling' pushes gold to 4-week low)

"What's happened? Peace, or whatever, has broken out. And peace—or at least a lessening of the discord—is always bearish of the gold market," Gartman said.

Whereas U.S. military action once seemed likely in Syria, the emergence of a credible diplomatic option has changed the odds completely. And since war-based uncertainty tends to boost gold, the smaller chance of a strike took a serious toll on it.

(Read more: More 'innocent victims' if US strikes Syria: Putin)

Gartman also pointed out that gold had been a crowded trade, saying, "A lot of people were bullish, a lot of people were long."

Finally, he said, expectations that the Federal Reserve will taper their quantitative easing program has become a massive concern for bullion.

"It appears to me the economy's doing better. That reduces the amount of accommodation that we think the Federal Reserve is going to push into the market."

(Read more: Keep calm and buy the taper: Pro)

All in all, buying gold toward the end of August turned into a "Murphy's Law" trade: Everything that could go wrong did go wrong.

"Throw peace on top of lesser accommodation on top of quietly reduced commodity prices generally on top of rising stock prices—all those things rolled into one great bouillabaisse says gold prices want to go lower," he said. "They have been going lower, and it's really getting quite ugly, isn't it?"

So instead of waiting for the trend to reverse, Gartman is now taking the other side.

When looking at the market today, "I actually don't like gold better than stocks," he said. "About a week and a half ago I started buying stocks again."

In a mid-august appearance on CNBC's "Closing Bell," Gartman said he was going to the sidelines on stocks after "terrible" trades in recent weeks.

—By CNBC's Alex Rosenberg. Follow him on Twitter: @CNBCAlex.

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