Check if a post includes a Gallery, and include a X-UA-Compatible to the head

functions.php

<?php
function jeherve_check_galleries( $post ) {
    global $post;
	
	if ( false !== strpos( $post->post_content, '[gallery' ) )
		echo '<meta http-equiv="X-UA-Compatible" content="IE=8" />';
}
add_action( 'wp_head', 'jeherve_check_galleries' );

Note: when WP 3.6 will be out, you can use has_shortcode instead:

function jeherve_check_galleries( $post ) {
    global $post;

    if ( has_shortcode( $post->post_content, 'gallery' ) )
        echo '<meta http-equiv="X-UA-Compatible" content="IE=8" />';
}
add_action( 'wp_head', 'jeherve_check_galleries' );