How do I find all gem dependencies?

I am looking to deploy my rails app into an environment that I don’t
control. I don’t want some (future) change in gems to break the app.
So I am using

rake rails:freeze:gems

to freeze the rails version and

rake gems:unpack
rake gems:unpack:dependencies

to freeze the gems in use.

As I understand it this will make the app use the versions of code in
vendor/ and it doesn’t matter what gems are added/removed/upgraded on
the server. Is this correct?

Now to my real question, ‘rake gems:unpack’ only works for gems listed
in config/environment.rb. How do I find gems that are in use by my
app, but are not listed there? I know all gems should be listed, but
I didn’t always know that and I still need to future-proof my older