did you check the server status? you can do this with 'tabadmin status'. this is where I would start to verify that the gateway is running.
From the URL in the screenshot, it appears that you have configured the gateway port to 8000 during installation. Is it correct?
You can verify this from "Configure Tableau Server" window from "All Programs" on your computer.
If you have not configured port 8000 and the port on the "Configure Tableau Server" window appears as 80 then you may lauch the URL as http://localhost/
However as Jeffrey suggested, you may first verify the server status if the gateway service is running or not.