Using the custom movement plugin, simply "accelerate" (through the custom movement --> set speed action) towards whatever point you want. It's possible to simulate complex gravity behaviour (eg, multiple points of gravity) with this method.
Edit: Here's an example where using families, you can achieve this:
> + System: For each spaceyobject
-> Projectile: Accelerate speed : spaceyobject.Value('Mass')/((distance(spaceyobject.X,spaceyobject.Y, Projectile.X, Projectile.Y))^2) towards spaceyobject
Where you can have multiple objects with the family attribute "spaceyobject". The projectile will accelerate towards "spaceyobjects" according to the equation: (mass (arbitrary private variable held by all spaceyobjects))/(distance from projectile to object^2)
Because this occurs for each spacey object, you can achieve quite realistic (within reason) behaviour. You'll need "mass" values of somewhere around 25,000.
I feel like an idiot.
When I added custom movement to a sprite, and saw there were no options to change, I assumed it was not ready yet. Never occurred to me that they were all assignable actions.
Guess this is why I could never understand programming languages.
Thank you so much, while this does not affect my gravity issue just yet (as I will have to figure out how this all works), it has allowed me to set up movement the way I want.