I think the online build service of GDevelop5 needs some clarifications:
You can create your entire game in GD5 without ever using the build service. This is completely optional and only helps with packaging your game as an Electron App for PC or a Cordova App for mobile platforms. The free plan lets you create and download a package two times per day. Personally I haven't reached that limit yet, since you probably won't publish a game or update it more than once per day and running a preview works without the build service.
The export dialog in GD5 lets you export your game locally without using the online services at all. If you are working on a pure HTML5 game, you can just do a local export and upload the files to your webserver. If you want to release for PC or mobile it is also possible to export your game as an Electron or Cordova project on your local PC (see the manual export options in the wiki). But in this case you need to install the Electron/Cordova dependencies yourself and build the app on your local computer. But since this process isn't straight forward and GD is aimed at novices in game development, the online build service was created to make it easier for them. But since no one lets you host the whole tool chain for free, the developer of GD needs to charge some money for the service. (which isn't very much compared to the commercial competitors).
TL;DR: You don't need to pay anything to use GDevelop5. All features of the IDE are available without any subscription. The only thing that costs money is using the "one click online build" service for Cordova/Electron Apps more that twice per day. But this can also be done on the local PC of the game developer. (after installing the tool chain and reading some tutorials).