It seems to me like the only way to really provide any sense of security is to force gems uploaded to RubyGems to be signed. There is some discussion here ( https://github.com/rubygems/guides/pull/70 ) about why the Rubygems PGP CA isn't really worth using in its current state. As we've seen with Javascript dependencies, we can only put off dealing with this problem for so long.
It's staggering the lack of consideration given to basic security by what should be competent software engineers.