You’ll need to add the gem to the gemfile. Some of the other guides may have better instructions for that part of the process,so you might look at some of those. The gem should be just “mysql2” though, I’d think.
It’s hard to keep topics like that up to date, and those who do imports regularly don’t look at them. And little details that trip up a novice are invisible to those who are familiar with the various pieces many of which (like installing mysql, importing the database, setting up a user in the database, installing gems and doing a bundle install} are not really discourse issues.
And then, when someone like you does manage to figure out out, you’re too busy at the end to spend another couple of hours tweaking these instructions.
If you have a budget, I could start offering an installation service that included mysql, the proper gems installed and a script that will kick off the import.