I manage the refresh of several datasets. At least once / week, I end up in a situation where one of my datasets isn't refreshing, and I see that the data gateway has been "deselected" for lack of a better term. When this happens, I can expand the list of "Datasources included in this dataset", and complete all of the drop-down mappings on the right side, however the "Apply" button at the bottom never becomes selectable, so I cannot save my changes.
When this happens, I have to have someone else momentarily take over ownership of the dataset. After that has been completed, I can take ownership back, and it will allow me to complete the configuration of the dataset. This is challenging because it makes me reliant on finding someone else to take over the dataset to move it back/forth between two people. There's other people that have had this issue, and their solution was to upload a blank PBIX with the same name, and then re-upload the PBIX essentially re-creating the entire dataset.