Gdevelop depends on Cordova plugin called "cordova-plugin-admobpro" this plugin is Not Free. The plugin will automatically take 2% off revenue once your app makes more than $1000. If you do not want to take 2% revenue you can buy a license for $20. You can read more about the license.
As Google AdMob does not disclose how much revenue an app has made. This plugin will estimate when you app will make $1000 and then take 2% revenue. All of this sounds very unclear to me. Also, there is no saying if the revenue will be increased form 2% to 10$?
On the other hand, there is a very good alternative Free and open source plugin with a lot of great features and best of all with no hidden fees or any sort of revenue deductions, check it out on the following link: https://github.com/ratson/cordova-plugin-admob-free
It will be great to replace the current plugin with this one