Thank you for the opportunity to help you with your question!
it helps download and install Ruby gems under the local Ruby installation. these are the steps to install;
open the gem manager dialog box.
In the Install Gems dialog box, decide the needed gems in the list. For each gem, you can select its version to be installed. If the version is not defined, RubyMine will automatically take the latest one.
If the desired gems are not found, you can look in a different repository. To do that, clickManage gem repositories button, in the Edit Gem Repositories URLs dialog box specify the desired URL, and click [img alt="add" src="https://www.jetbrains.com/img/webhelp/idea/new.png" class="inline-icon-12">. Back in the Gem Manager, click to make visible all the gems, located in the specified repositories.
Specify whether you need to install dependent gems automatically. Be default, this option is selected.
Please let me know if you need any clarification. I'm always happy to answer your questions.