There is no gravity in the Visionaire universe because the engine was never intended to be used for other game genres than point & click adventure games. In Game Maker Studio for instance you draw masks over the top of your scene tile assets to show what is solid & what is not. Visionaire only has borders & paths, although we do have an action area system, that you could possibly use to define solidity. If player inside action area gravity = 0 else gravity = -1 or whatever. Unfortunately this is theory on my part & you would need to use Lua script either way to create it.
As the engine currently stands, there is no reason for Simon to bother attempting to implement the box2d physics engine. As I said though, that might change at some point in the future, but I have no idea when that will be.
The only thing I can tell you is that it is theoretically possible to do what you are asking, but most of it will require you to code it with Lua script, sorry.