Adapted from: http://code.hyperspatial.com/1534/add-class-to-gf-submit-button/

gistfile1.php

<?php
/**
 * Override the output of the submit button on forms, useful for
 * adding custom classes or other attributes.
 *
 * @param  string $button An HTML string of the default button
 * @param  array  $form   An array of form data
 * @return string $button
 *
 * @filter gform_submit_button
 */
function fjarrett_gform_submit_button( $button, $form ) {
	$button = sprintf(
		'<input type="submit" class="my-custom-class" id="gform_submit_button_%d" value="%s">',
		absint( $form['id'] ),
		esc_attr( $form['button']['text'] )
	);

	return $button;
}
add_filter( 'gform_submit_button', 'fjarrett_gform_submit_button', 10, 2 );

Great!
And add classes for text input fields?
Normal is the"length" of the field, medium, small, ...