Auto-complete WooCommerce order based on payment gateway

auto_complete_order.php

<?php

function auto_complete_order( $order_id, $old_status, $new_status ) {
    $order = wc_get_order( $order_id );
    /*
     * You can get the `your_payment_gateway` identifyer by visiting the WC payments setting page 
     * and selecting the gateway you would like to use. The payment gateway identifyer is the "section=" part of the browser URL
     */
    if ( $new_status === 'processing' && $order->get_payment_method() === 'your_payment_gateway' ) {
        $order->update_status( 'completed' );
    }
}
add_action( 'woocommerce_order_status_changed', 'auto_complete_order', 10, 3 );