I have received the refund email, which shows that the refund has been completed. However, I did not receive the refund at all. After checking my payment platform, I said that the GOG platform did not refund me at all. Now it is useless to send messages with the customer service. May I ask what should I do? They also deleted my game, but I did not receive the money either!

Why do you think it is useless to talk with GOG support again? Just say that you didn't receive the refund and ask to have a look why this could be.

The problem is that the customer service staff ignored me. I waited for several days and sent numerous emails, but they merged my emails into the refund. I didn't receive them at all.

If the GOG platform gives me a refund, My payment method will be received immediately. This only shows that you did not give me a refund, and pretend that the refund has been completed to delete my game!

Is there any customer service that can help me with that?

Only Gog's customer service can help you.
They don't reply immediately due to this:
https://www.gog.com/forum/general/expected_delays_in_support_replies

So customer service won't reply to this message? Then I contacted customer service through various means and ignored me. Why do I believe this platform, the attitude is really bad!

try with your bank support

I checked my payment platform and they said there was no refund at all
I have been waiting for several days, but I haven't received the refund you said, so you just marked that it has been solved. And how many times have I sent them to me since they were merged, and the game has been removed? I have nothing now! No games and no money! I hope you can make it up to me

Good luck. Perhaps a GOG staff member sees this post and can help you.

Also keep in mind we entering one of the longest holiday periods (Christmas to New Year's) so most places are working on revised hours and staff.

You don`t close it, only mark one answer as solution, so others might find that solution too.
Good to read, that your money is finaly there. Merry X-mas. :-)