Bootstrap 3 input group for Gravity Forms (only tested with 1 input field and 1 submit button)

gf-input-group.less

/*
 *
 * Bootstrap 3 input group form for Gravity Forms
 *
 * Use with a class assigned to the form
 * or on .gform to affect all Gravity forms
 *
 * Example 1 (affects targeted form class only): .my-form-class { &:extend(.gravity_form_input_group all); }
 * 
 * Example 2 (affects all Gravity Forms): .gform { &:extend(.gravity_form_input_group all); }
 *
 */

.gravity_form_input_group {
	&, &_wrapper {
		form {
			&:extend(.input-group);
		}
		.gform_body input {
			&:extend(.input-group .form-control, .input-group .form-control:first-child);
		}
		.gform_footer {
			&:extend(.input-group-btn);
			vertical-align: bottom;
			input[type=submit], button[type=submit] {
				&:extend(.input-group-btn > .btn, .input-group-btn:last-child > .btn);
			}
		}
	}
}