When you modify the Gemfile at runtime, it means that it will also add/change things in the Gemfile.lock. That means that the dependencies/versions that you’re using in production don’t match the ones we test Discourse against.
Interesting - I wasn’t aware of that. I suppose it’s much lower risk for the ‘import’ templates, since they’re only used for a short period of time during a site migration. Even so, I still wouldn’t recommend using this technique in a ‘production’ template - any of the other three options would be much cleaner.