add-product-field.php

function woo_add_product_gender() {

    echo '<div class="options_group">';
    woocommerce_wp_select(
        array(
            'id'      => '_product_gender',
            'label'   => __( 'Gender', 'woocommerce' ),
            'options' => array(
                'male'   => __( 'Male', 'woocommerce' ),
                'female'   => __( 'Female', 'woocommerce' ),
                'unisex' => __( 'Unisex', 'woocommerce' )
            )
        )
    );
    echo '</div>';
}

function woo_add_product_gender_save( $post_id ) {
    $woocommerce_dropdown_field = $_POST['_product_gender'];
    if ( !empty( $woocommerce_dropdown_field )) {
        update_post_meta( $post_id , '_product_gender', esc_attr( $woocommerce_dropdown_field ));
    }
}
// Display Fields
add_action( 'woocommerce_product_options_general_product_data', 'woo_add_product_gender' );

// Save Fields
add_action( 'woocommerce_process_product_meta', 'woo_add_product_gender_save' );