ApplePay is simple a middleman in the payment system. They don't actually have your money at any time. They just interface between your payment method and the vendor. The only time they would play a role in this would be if you had subscribed through Apple, but you say you do not see this in your purchase history.
https://support.picsart.com/hc/en-us/articles/360002232017-Subscribing-to-Picsart
I am not clear as to what you mean when you say the Gmail used to buy the subscription is different from your Apple Account. If a different account purchased this then any payments made through Apple Pay would have been through that account, not yours. Do you have multiple accounts?
Refer to this page for Apple Support features ➔ Contact - Official Apple Support
Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page under “Get more help” (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.
This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple Support - Apple Support For some countries look under “Other” listing at the end of the list. Not every country has telephone support, and phone numbers may only work when calling from the same country.
