Are you able to start Apache from the command line? I’m curious if there’s an error occurring of some kind, or if Virtualmin is just having trouble interacting with Apache.
I suspect you’re seeing some configuration issues that are specific to Gentoo (note that we recommend CentOS, Ubuntu, or Debian).
However, digging down into that problem you’re seeing – as I look at the Virtualmin code, it appears that error is thrown when Virtualmin runs the Apache command to test the config, and it returns an error of some kind.
If you look in Webmin -> Servers -> Apache -> Module Config -> System Config, you may want to verify the various options you see there.
In particular though, what is “Path to the apachectl command” set to? If that’s set to an invalid path, it’s possible that could cause that part to fail.
Also, if you go into Virtualmin -> System Settings -> Re-Check Config, does it detect any problems?