don't even try to use gmail for smtp etc.
one of the things you are paying hostgator for is the use of the email servers for this type of thing.
simply do as stated above. go to your hosting control panel, create email addresses for each board or just one for all;
use that email address for all the settings for all the boards.
in your email settings, chose "email" don't choose smtp.
If you want to use your gmail account for checking the emails then you can do a simple forward in your hosting control panel so that all emails to email@example.com
get sent on to your regular gmail account.
( don't know why anyone would want to use gmail when they have their own email server with their own domain name, but hey..... )