That suggests a simple solution to the problem that can be done on the gmail side: for any small mail server (to pull a number out of my behind, say less than <200 emails a month to any @gmail address for the last 12 months), white list them if they satisfy the rest of the usual requirements.
Even if someone attempts to game the system and creates a number of servers, for an effective spam campaign it means a large number of servers, costs go up.
It might be worth experimenting with "abusing" this behaviour to put your small server on the Gmail whitelist. Start sending a large number of generated emails to a @gmail.com mailbox, log in and ensure none of it ends up in spam (ideally automate that too :), and there we are. Anyone have any idea how many emails that is? :)