I got through the opening by testing and tweaking the gravity and jump power. I went a bit higher on the gravity and lower on the jump (I think it was in the 150-250ish) range. I didn't recognize that the graphics were default--I don't have much experience with GDevelop. I do agree with you that adding their own graphics would be a great step in polishing the game; the assets don't have to be perfectly crafted but doing their own art or partnering with an artist is very important for making finished product--especially on future projects!