First thing, make sure to have an account on Facebook Developers. Create an app and configure it to choose "Games" as its category and enables Instant Games in Products:
In GDevelop, open your game and then the Export window. Choose Facebook Instant Games, click on Export and wait a few seconds.
Go back to your game page on Facebook Developers, choose Web Hosting in the menu and upload the file that was exported by GDevelop.
You can also add testers to let your friends try your game.
Now, get your phone and open Facebook. Open the Gaming section, and click on the icon of your game to start playing to it!
GDevelop gave you access to Instant Games specific features, like the name of the player, to better integrate your game in Messenger conversations.
When your game is ready, submit it for review by Facebook so that it becomes available worldwide!