The system.net section settings are no longer used so only the other settings matter.
Note that settings in user.config will trump the ones in web.config
I'm not aware of being able to use custom domains with gmail. I would expect that you would have to use the gmail address in settings and as the from address in site settings.
I really don't think gmail is a good solution for web site email, your mileage may vary on that and they may limit the amount of mail you can relay.
Hope that helps,