I had looked at the Announcement Mailing List when I first started, but it doesn't do what I need. Anyway, it looks like getting the SPF record set up correctly fixed the problem. It seems DKIM is set up automatically.
One possible issue is that according to this SPF validator, the SPF record should also be published as type SPF, but there's no option in the Dreamhost control panel to choose SPF as a type, only TXT (and some others that aren't relevant here).
But anyway, I'm not getting the softfail from Gmail anymore, so I guess it's OK for now. (Ironically, in the midst of all this, I had an email from Dreamhost itself tossed in my Gmail spam folder, supposedly for "phishing", even though it was just a payment receipt. The false positives are maybe getting a bit out of control....)