I'm fairly certain Native Americans found some gold and traded it in Forest Grove and elsewhere. I'm having a hard time believing it came from that area though, or was "mined" there, which throws the whole story into questionable range. Story doesn't name names, or even the tribe from which these alledged gold came from: very unusual. The Tchinook tribe further north along the Columbia River and at least a part of the Willamette River, did use salvaged gold coins as markers in a gambling game they played. There were some gold coins found in Indian graves uncovered during highway construction in Pacific County, Washington. So it is possible.
But in Forest Grove the Grand Ronde tribe more likely, and not recorded to have used gold for gambling or any other purpose.
Combined with the lack of known gold-producing areas nearby, makes this legend unlikely, I think. Not enough fact to warrant further research.