@istudent This is kind of a tricky question in that there are so many possibilities for implementation. First things first, there is no concept of sharing a datasource without hooks. What I mean by that, is that you could create an organizational content pack and share the dataset, but you would still own the original, and everyone is just creating copies. (gets ugly)
It is normally recommended that you start in the Desktop, this offers you the most flexibility to alter/change connections, publish to any workspace, and have a back up of reports. If you develop everything in the Service, your reports/dashboards are stuck in 1 workspace, and there are limitiations on what you can do to the dataset (limitations = you basically can't do anything). I would foresee end users wanting to create their own calculations etc. -> This requires the Desktop.
Granting access to the gateway datasource to a user just gives them permission to use the gateway if they deploy that datasource.
The most straightforward thing I can think of is to create a Power BI Desktop Template with the appropriate data source connections. Make sure the end users have permission to see it, and then either send the template or put that in a location that people can cut copies from to start their reports...
Let me know if you have any questions, there is a lot more info that can be embellished on as far as best implementation strategies.
