Well for starters you may want to try not using GDevelop on your phone, as it was not designed to function on one. The game is stretching to fit your screen, which is much longer than the resolution area. Try modifying these settings under game properties:
Try changing the
Change the width to fit the screen or window size to
No changes to game size. If that doesn’t work try un-ticking the box
Update resolution during the game to fit the screen or window size. Also, you can just adjust the resolution width/height to change the resolution area in the scene editor (by default they are 800/600).
Hope that helps!