Autocomplete paid orders for specific payment gateway in WooCommerce.

autocomplete-paid-orders-for-specific-payment-gateway.php

add_filter( 'woocommerce_payment_complete_order_status', 'wc_auto_complete_paid_order', 10, 3 );
function wc_auto_complete_paid_order( $status, $order_id, $order ) {
    if ( $order->get_payment_method() === 'stripe' ) {
        $status = 'completed';
    }
    return $status;
}