You should not need to use the SSC. The Gateway is a stand alone application and is not federated into the SSC's WebSphere cell which means it is not it's deployment manager. Registration with the SSC allows you to do the things you mentioned from within the SSC as opposed to in the Gateway's ISC.
Regardless of whether you have registered with the the SSC open D:\IBM\WebSphere\STGWAppServer\profiles\RTCGW_Profile\logs\AboutThisProfile which will look like:
Application server environment to create: Application server
Disk space required: 200 MB
Profile name: RTCGW_Profile
Make this profile the default: True
Node name: *************
Host name: *****************
Enable administrative security (recommended): True
Administrative console port: 9061
Administrative console secure port: 9044
HTTP transport port: 9081
HTTPS transport port: 9444
Bootstrap port: 2810
SOAP connector port: 8881
Run application server as a service: False
Create a Web server definition: False
Performance tuning setting: Standard
You want to put together a URL such as https://node.acme.com:9044/ibm/console
and then log in with the admin username and password you created at install time eg wasadmin. This will take you to the Integrated Solutions Console.
With regards to "unable to connect to data source" you mention in
I have seen this before but if you are going to use the ISC on the Gateway then this can be ignored and the Gateway unregistered.
Try the ISC URL, does this allow you to log in? You will be able to see the same self service test and configuration sections as you see in the SSC.