See https://gist.github.com/richardvav/6947682

<?php
// UPDATE: see https://gist.github.com/richardvav/6947682 up to date October 20, 2013
// Go there now and ignore the rest of this code
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// Help Scout 30310 Richard Vav
// improved September 19 2013
// to exclude a field from notifications assign the field the CSS Class Name gf_exclude and then use the {all_fields:exclude} merge tag
add_filter( 'gform_merge_tag_filter', 'exclude_from_all_fields', 10, 4 );
function exclude_from_all_fields( $value, $merge_tag, $options, $field ) {
    $options_array = explode( ",", $options ); //breaks options into an array
    $exclude = in_array( "exclude", $options_array ); //returns TRUE if 'exclude' is found in the array
    $class_array = explode( " ", $field["cssClass"] ); //breaks the fields CSS classes into an array
    $exclude_class = in_array( "gf_exclude", $class_array ); //returns TRUE if 'gf_exclude' is found in the array

    if ( $merge_tag == "all_fields" && $exclude == true && $exclude_class == true )
        return false;
    else
        return $value;
}