As an AA gold, you have OneWorld Ruby status.
As a OneWorld Ruby, you can use business class check-in. The chart 'Non-NonRev' linked to is correct.
At least in BOS, the sign for the business class check-in shows the image of OneWorld Ruby, meaning you can use it.
If this happens again, just tell the agent that you are OneWorld Ruby (being AA gold means nothing to them even though that is what makes you Ruby). I suspect the vast majority of agents know the rule, but you could carry around the chart to show them if it is a frequent problem.