Existing PowerBI installation - how to find/modify the SQL query for a table?

Hi , IanMurphy 

According to your description, you noe have a dataset in Power BI Service , but you need to modify a M query for a table in it .

If this is the case, you need to download the pbix file of the dataset, then open it locally, and download it in Power Query Editor.

For more information, you can refer to:
Download a report from the Power BI service to Power BI Desktop - Power BI | Microsoft Learn

And then you can modify it in your on-premise conmputer and then you need to republish the dataset and configure the refresh of this dataset again.

And you also can see the datasouce by "View Lineage" of the dataset:

And we can see the gateway configuration in the setting of the dataset if you have the permission.

By another , the data gateway just a service or application to keep the connection for your on-premise and Power BI Service so that the power BI Service can get the data from your data source.

For this , you can refer to :
On-premises data gateway - Power BI | Microsoft Learn

Thank you for your time and sharing, and thank you for your support and understanding of PowerBI! 

Best Regards,

Aniya Zhang

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