GDevelop game engine and game editor are indeed free to use and open-source, under the popular MIT license.
The GDevelop company, founded by GDevelop core team members, have been building online services to easily build, publish, operate and monetise games made with GDevelop.
By getting a subscription, you get access to these services, which are perfect to take your game to the next level, work on larger projects and reach out to a large audience of players.