Hey Lucy. As I understand it, Marketo has a dedicated IP address feature that puts you on your own domain. It costs money and you also have to have a pretty high volume database to qualify for it. You can chat with your Account Manager about that process.
We were having the same issue with being blacklisted by some actual clients of ours because of the shared IP address through Marketo. That's why we made the switch.
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You can have Marketo relay mail through your own mail servers. It's considered an add-on and, as I understand it, quite costly.
You would not want to (nor would you likely be allowed to) use Gmail as that relay. It's expected that you route through servers you completely control (and whose presence on blacklists you *also* control).
Outside of the official support, I've engineered a solution to send only selected mail via a relay, for example to known customer domains. Such a project is not for the faint of heart.