gmail.com supports fetching email from external servers via POP (it's under settings, accounts). His users could use that and skip most of gmail's spam filtering completely. It doesn't (reliably) support forwarding mail to it without using some list software in the middle. I believe if jwz set up mailman or something similar, it wouldn't have a problem with that, because mailman would properly re-write the envelope sender as the list address.

You can add external POP accounts, and I think that would be the solution for OPs problem.