If you just installed Gentoo on WSL you probably can't login as root by executing 'su' as your password will always fail.
An easy trick is to run 'lxrun /setdefaultuser root' on your cmd or powershell before opening your gentoo bash. After, you should delete the user you created on ubuntu on WSL and create it again using your gentoo bash in order to be able to login as the regular user. Then 'su' will run successfully accepting your password.
If you just installed Gentoo on WSL you probably can't login as root by executing 'su' as your password will always fail. An easy trick is to run 'lxrun /setdefaultuser root' on your cmd or powershell before opening your gentoo bash. After, you should delete the user you created on ubuntu on WSL and create it again using your gentoo bash in order to be able to login as the regular user. Then 'su' will run successfully accepting your password.