Maybe it should be a Bundler plugin?

[choosen]

You can write your own bundler plugins instead of invoking gem.
did you consider that ?
more info:
https://bundler.io/guides/bundler_plugins.html

I also think about those bundle update switches:

--minor
Prefer updating only to next minor version.

--major
Prefer updating to next major version (default).

--strict
Do not allow any gem to be updated past latest --patch | --minor | --major.

--conservative
Use bundle install conservative update behavior and do not allow indirect dependencies to be updated.