These are classes defined in the Gravity Forms style-sheets that allow for more complex layouts. These classes can be assigned under the advanced tab, for the field labeled “CSS Class Names”.
For two column layouts use:
gf_left_half
gf_right_half
For three column layouts use:
gf_left_third
gf_middle_third
gf_right_third
List classes (turn multiple choice/checkbox into multiple columns)
gf_list_2col
gf_list_3col
gf_list_4col
gf_list_5col
gf_list_inline
There are a few other options on the referenced url, but I have not had an instance to use these yet.
reference: http://www.gravityhelp.com/css-ready-classes-for-gravity-forms/
Update:
I’ve found these ready classes extremely useful when using gravity forms with out any custom styling. I’ve also found an extremely useful plugin to make assigning these classes easier for me and my clients.
Check out Gravity Forms CSS Ready Selector plugin. It add a button next to the ready class field to insert the class for the layout that you want on the form.