I couldn’t produce a better tutorial than their own, so please see it here: http://www.gravityhelp.com/documentation/page/CSS_Ready_Classes
Otherwise, here is a simple list of the classes you can use to customize your form:
gf_left_half (2 Columns)
gf_right_half
gf_left_third (3 Columns)
gf_middle_third
gf_right_third
gf_inline (Fluid One Line)
List Classes:
gf_list_2col
gf_list_3col
gf_list_4col
gf_list_5col
gf_list_inline
gf_list_height_25
gf_list_height_50 _75 _100 _125 _150
gf_scroll_text (This converts a section break field into a box with a fixed height that will automatically show a scroll bar if there’s a large amount of text.)
Update: I just learned of a plugin that makes it easy to use these ready classes by adding a popup window. I think adding these classes is already pretty easy, but if you don’t want to have to remember them or continually refer to them, this plugin makes it much easier. Gravity Forms CSS Ready Class Selector
