It's a nice puzzle game. The code does show the idea of the game and how it could be implemented.
However, the way it's been coded (a different variable, cell and image for each tile) means it's not best practice or easily extended. To add more rows or cells means adding more events and objects for each new tile.
I think this would be ideal as an example program if you'd consider a refactor using arrays and instance variables on the cells, tiles and possibly select object. It would reduce the project down to a dozen objects and a lot less code.
After all, your game is simple in concept and events, yet fun to play. Great qualities for an example GDevelop game to learn from.