If you use the new Page Heros in GPP 1.7, you should be able to use a function like this:
add_filter( 'generate_page_hero_background_image_url', function( $url ) {
if ( function_exists( 'is_product_category' ) && function_exists( 'get_woocommerce_term_meta' ) && is_product_category() ) {
global $wp_query;
$cat = $wp_query->get_queried_object();
$thumbnail_id = get_woocommerce_term_meta( $cat->term_id, 'thumbnail_id', true );
if ( $thumbnail_id ) {
$url = wp_get_attachment_url( $thumbnail_id );
}
}
return $url;
} );