I have been unable to get this to work, and no response from MS on another thread of users having the same problem.
I need to know if this is hopeless, or I should continue pursuing.
I can connect to the gateway, but any access of SharePoint lists results in "401 UNAUTHORIZED", as many others have experienced. There is no definitive answer I can find as to whether this should work or not.
My SharePoint 2013 installation is accessed via https through a KEMP load balancer/reverse proxy.
I think your Asure AD that holds the PowerApps creds needs to sync to your on prem domain AD user base. I have accessed on prem SP 2013 for apps (so no, it is not hopeless) , but I am not a security admin, so I don't know the details of how to sync them up, to grant the rights to the cloud id.