So here's basically how the gateways work, and why mine didn't work.
You develop in desktop by first connecting to a server, in my case on a different machine, on a different network, but reachable by an IP/port, e.g, 50.60.70.80,9999, database name, and SQL credentials.
When I tried to publish to a web server also hosting a gateway, PBI would give the gateway those credentials (50.60.70.80,9999, DB, and SQL credentials). however, due to the firewall port forwarding, the gateway couldn't connect locally to that SQL database.
BUT, when I used another machine as the gateway (foreign to the SQL server), where that connection string could work, when publishing, PBI sends the credentials to the gateway, gateway says, "sure, I can connect to that", and pop! Worked easily.
(I had also explicitly added those credentials in apps.powerbi.com as part of my debugging to make a new data source on that gateway, so that may be a necessary prerequisite).
I hope this helps somebody in the future.
