Enterprise Gateway broke all my credentials

You can delete an Enterprise Gateway via powerbi.com website. It is not enough to simply uninstall the Enterprise Gateway software from the on-prem server.

Once logged into app.powerbi.com, go to settings (the little cog in the upper right corner) and choose [Manage gateways]. Then you can click the very small difficult to see (because of the light grey on white color scheme) dot-dot-dot next to the gateway you want to delete and choose [Remove]. I believe you need to be designated as an Administrator for the Gateway in order to complete this.

---- background that may help with your other issue ----

I discovered this after quite a bit of fiddling around. Another developer had apparently configured a gateway but it wasn't working correctly. I uninstalled the PBI-EG application on the on-prem server and reinstalled it - thinking PBI.com would pick up the change. However, when I went to configure the new gateway during the installation process, I was greeted with an error that the name (PBI_GWAY01) was already taken. So I gave it another name (PBI_GWAY02) and moved forward. However! After publishing a basic report from PBI-Desktop, I was met with more gumption traps after attempting to open the report. It seems during the publishing process, the report associated with the first gateway (PBI_GWAY01) and a connection could not be established (because I'd uninstalled it from the server). There was a small ray of hope: the error message was accurate ("PBI_GWAY01 appears offline") and even included a link to allow me to choose another gateway. I clicked the link expecting to see PBI_GWAY01 and PBI_GWAY02 listed with the option to select PBI_GWAY02 (which I knew was online). However, the only gateway displayed was PBI_GWAY01 and it was marked as offline. :-(  

I'm not sure what to make of this - still waiting on the other developer to return so they can delete PBI_GWAY01 from PBI.com to see if that clears this up (I'll return to update once I know more).