itch.io

For more immersive skyboxes I recommend going to Poly Haven for their pure sky hdri's.

This: https://polyhaven.com/hdris?s=pure%20skies

They're free, Under CC0 License , HD,  and beautiful.

1. Click on one you like
2. Click the three bars beside the download button
3. Click 8K Tonemapped JPG (This should open a new tab with image)
4. Right click and and choose save as. 
5. Import it into GDevelop
6. Enjoy the majesty