functions.php

<?php
/**
 * Grid Content
 * Change the number of words in excerpt if in the grid loop
 */
function be_grid_content() {
 
  // First, we make sure we're in the grid loop.
	if( ! be_grid_loop_pagination() )
		return;
 
	// Change length if teaser
	if( in_array( 'teaser', get_post_class() ) )
		$length = 20;
	else
		$length = 55;
 
	echo '<p>' . wp_trim_words( get_the_excerpt(), $length ) . '</p>';
}
add_action( 'genesis_post_content', 'be_grid_content' );
remove_action( 'genesis_post_content', 'genesis_do_post_content' );