After installing and configuring the sfgae application from the UI I keep getting "Unable to gather configuration data from splunk" in the log.txt.
Am I missing some part of the setup?
That error code is thrown when the application is unable to gather your stored App Engine credentials from the Splunk instance. This is likely caused from an app that hasn't run through the setup process yet. If you head to /manager/sfgae/apps/local/sfgae/setup and follow the setup procedures, this should remedy your issues.