Tableau Tech Support will need to look at your log files to determine why the gateway process is down, I suggest opening a case with them and providing your logs files.
In my case permissions of key file were the reason: user running Tableau Server didn't have read access to the key.
Anyway, you can check /var/opt/tableau/tableau_server/data/tabsvc/logs/httpd/startup.log to see why Gateway (which is Apache web server actually) doesn't start.
Have a look in the Gateway logs (httpd) to find the root cause : tsm maintenance ziplogs
When Tableau Server is configure for SSL and the Gateway is down, it's most of the time because the SSL certificate didn't have the requirement.
Please have a look on the following article for the requirement : https://onlinehelp.tableau.com/current/server-linux/en-us/ssl_config.htm