The most common pattern is to use Rails Engines for that matter. Rather than interacting with a real database, Rails engines would allow you to replicate the functionality of ActiveRecord::Base (and other Rails classes) by injecting its behaviour into your gem. By doing so, you can test the implementation without the need for an actual database connection. By doing so

One of the most popular use cases for that can be seen on Devise codebase. You can check some examples here.

For more info see: Getting Started with Engines docs