Data source error: {"error":{"code":"DM_GWPipeline_Gateway_MashupDataAccessError"

Hi. A mashup error usually means that the Power Query executed for the semantic model has an error. Ir order to capture the error, the best approach would be openening the pbix file at the same server of the gateway and refresh it. The refresh will show a better error details to understand the problem.

I hope that helps,