add_filter( 'gform_field_value_leadsource', 'gform_populate_leadsource' );

function gform_populate_leadsource( $value ) {

    if(isset($_COOKIE['wpf_leadsource']) && is_array($_COOKIE['wpf_leadsource']) && isset($_COOKIE['wpf_leadsource']['leadsource'])) {
        $value = $_COOKIE['wpf_leadsource']['leadsource'];
    }

    return $value;

}

add_filter( 'gform_field_value_utm_campaign', 'gform_populate_utm_campaign' );

function gform_populate_utm_campaign( $value ) {

    if(isset($_COOKIE['wpf_leadsource']) && is_array($_COOKIE['wpf_leadsource']) && isset($_COOKIE['wpf_leadsource']['utm_campaign'])) {
        $value = $_COOKIE['wpf_leadsource']['utm_campaign'];
    }

    return $value;

}