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1. Does that mean I will have to get an account for each of my domains and default those email accounts to the domain where GMail resides?
It depends on how you configure your mail server. Typically, you would configure a catchall account for each domain.
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2. Why do I need to point the MX records for those domains to the same server? Shouldn't they be pointed to their own domain since their email account's email is received then send to domail where GMail resides? Please explain as I am confused.
Again, it really depends on your set up. You should point the MX records for the domains to the mail server that's configured to accept mail for those domains. Depending on your set up, it could be on separate machines, or on a single machine. On Gossamer Mail's side, it doesn't care where the mail servers are as long as it can fetch the mail from the catchall account using POP3.
Adrian