TP's Force operator can be used to simulate gravity.
You can simply put a negative number as the force to make objects fall down (-Z).
You do not need to specify a Direction. Negative values indicate down in this case.
When using Max units where 1 unit = 0.1 Meter, and calculating distance over time with:
d = (9.8066 * 10.0) * ((frames / 24.0)^2) / 2
Note: (mult 9.8066 by 10.0 to convert units to meters, and frames/24.0 to get distance in frames) (set Max units where 1 unit = 0.1 meters)
To simulate Earth gravity with these units, TP's Force operator should have a value of about -98.1