It is possible for the publisher to sign the gems [1], but it's not common.
If rubygems.org is keeping fingerprints of each gem, then it still isn't sufficient, since those could have been compromised as well. If there's no other trustworthy source of fingerprints, then maybe we need to crowdsource it. Built a tool that will md5sum all the .gem files in your local cache directory, so that we can look for any files that were changed on rubygems.org