Domain Hosting is mandatory for the Teams GW service to be set up. (This is how we distinguish one customer's Teams GW Service from another's on the multitenant Cloud Service.)
If customdomain.com is the video domain chosen, the Microsoft Teams Virtual Receptionist for 2-stage dialing will be at email@example.com and 1-stage dialing into the Teams meeting will be <ConferenceID>@customdomain.com.
If you have soft clients and/or video endpoints with a URI address with the same domain as the Teams GW, these calling parties can be considered as "trusted parties". They will enter the Teams meeting without the need for a meeting manager to manually admit/deny entry through the MSFT Teams Lobby.