Gallery plugin forces itself above text

Your shortcode function callback must return the output, like this:

function gamma_gallery( $attr ) {
    $html="";

    // your stuff, where all the output is assigned to the $html variable

    return $html;
}

or you could try the output buffering:

function gamma_gallery( $attr ) {
    $html="";

    ob_start();

    // your stuff

    $html = ob_get_contents();    
    ob_end_clean();

    return $html;
}