If your pivot table includes a value summarized by COUNTUNIQUE it will always work in Geckoboard.

That means that if your current pivot table doesn't include a value summarized by COUNTUNIQUE, you can add a new value that is summarized this way. You'll end up with an extra column that might be useful too, or could disregard for the selection.

Another alternative is to add a new sheet to the same file and copy the values from the pivot table sheet to the new sheet (using any formula that can copy the entire content across). Then, you can use the new sheet in Geckoboard.