What will be the passcode for ATM? my GX bank code?
Yes, enter the same 6 digits PIN of your GX Bank card.
On the China's ATM, you select the CNY amount you want to withdraw, once you got your cash, you'll see your GX Bank's balance has deducted in MYR. Pretty straightforward.
The withdrawal limit is the same as your GX Bank's transaction limit settings.
https://help.gxbank.my/Personal%2F%2Fcards/..._the_GX_Card%3F

Can direct use GX bank without pair pair with Alipay?
Technically you can.
But don't be surprised if you see many shops there (e.g. restaurants, convenient stores, boutique stores) don't even have a card payment machine.
And even if there is, they may only accept UnionPay cards but not Visa/Mastercard cards.
But of course, if you so happen to see a Visa and Mastercard logo sticker/labels around the cashier, then you can be rest assured. 

GX Bank card uses Mastercard payment network.
So, as long that merchant accepts Mastercard card payment, then it should work just like any other Mastercard cards you have from other banks.
If you see a card payment machine on the cashier, you can just try and use your GX Bank card and wave it onto the payment machine to see if the transaction went through.
It may feels like a hit-and-miss thing, so do manage your expectation.

If you have other overseas-travel-friendly debit cards such as Wise card or TnG Visa card, it's a great idea to also bring it along as a backup. Don't rely 100% on GX Bank card only.

Even if that shop accepts cash, they may not have small change if you pay in larger bills.
Yes, China really embraces cashless payments till that extent.

Just received a notification from GX Bank to update my profile.
Only one question they asked to answer: Are you a Bumiputera?

Is everything ok with GX Bank lately?
Any hanky panky going on?