public Vector2 gravity { readonly get; set; }

In a similar way to how the _gravity works, the gravity vector here is scaled by time and applied as movement to the slide position. However, unlike _gravity, this has no way of increasing the velocity to produce an acceleration so if this is required then this should be maintained and the current accumulated gravity velocity passed in. The reason that gravity is separated from the provided is that it has a different behaviour in that it can produce slippage on surfaces where the angle is higher than _gravitySlipAngle.

NOTE: By default this is (0, -9.81). Using _zero results in no gravity being applied.

Additional Resources: Rigidbody2D.Slide, Rigidbody2D.SlideResults