I agree that it’s difficult to find technical help for bitsy now.
I am not familiar with the usage of the gravity hack and cannot help you there. Ladders in Cake climb were made with a tedious series of one-way exits: one leading down and one leading up for each level, with a single target/landing tile for the player to rest on in between. Moving backwards moved the player into the down exit and bitsy 3d tweens the movement directly to the target/landing tile on the level below without the tile the exit is actually placed on. This only worked because of the “dungeon-crawler” style controls of Cake Climb.