Looking at the application you can see its written "Access only for agents using the inctfj browser" so from this we can understand that access is only given to those who use the browser called inctfj.
So from our initial analysis we know we have to send a request to the server and the server should think that our request is coming from a browser called inctfj. For this we have to set the User-Agent header which is used as a characteristic string that lets servers identify the application, operating system, vendor, and/or version of the client the request is being sent from. To get the flag we have to set the user-agent header to inctfj using curl
curl -H "User-Agent: inctfj" http://gc1.eng.run:30783/ or any other tools/extensions.