Gravity will always work, unless you want to simulate something like the wild, swirling hair of a headbanger in outer space …
Gravity is a negative force since it is exerted opposite the Y axis (world coordinate system). The Gravity setting lets you define the direction and strength of the gravity.
If other forces (e.g., Cinema 4D particle modifiers) should affect the splines, they can be included or excluded using these settings.
Simply drag the respective object into the Forces field and select the appropriate mode for inclusion or exclusion.