<?php
//example.php

require __DIR__ . '/vendor/autoload.php';

use Payum\Core\PayumBuilder;
use Payum\Core\Payum;
use Payum\Core\Model\Payment;

$paymentClass = Payment::class;

/** @var Payum $payum */
$payum = (new PayumBuilder())
    ->addDefaultStorages()
    ->addGateway('aGateway', [
        'factory' => 'offline',
    ])
    ->addGateway('bGateway', [
        'factory' => 'offline',
    ])
    ->getPayum()
;

header('Content-Type: text/plain');

function getStorageExtension($gateway)
{
    $r = new ReflectionObject($gateway);
    $p = $r->getProperty('extensions');
    $p->setAccessible(true);
    foreach ($p->getValue($gateway) as $extension) {
        if ($extension instanceof \Payum\Core\Extension\StorageExtension) {
            return $extension;
        }
    }
}

$aGateway = $payum->getGateway('aGateway');
$aStorageExtension = getStorageExtension($aGateway);

$bGateway = $payum->getGateway('bGateway');
$bStorageExtension = getStorageExtension($bGateway);

var_dump($aStorageExtension === $bStorageExtension);
// true