If you open your Outliner, it gives you a list of all the objects in your scene. You can select it from there if you have one.

Gravity is not global. It is applied to whichever objects you have selected at the time of creating it.

If you created it without telling it to affect anything, you can do so under Window > Relationship Editors > Dynamic Relationships.

In this window, on the left, you see your Outliner list, on the right is the list of any sort major dynamics object depending on what selection mode you have at the top of the right side.

With fields selected, you see your fields. Select the objects you want the fields to affect on the left, then simply click the field on the right, turning it a yellow color indicating it is active on the selected object.