It was clearly stated "CAN BE VIEWED IN YOUR CREDIT CARD STATEMENT". You wait for your card statement to be delivered.
I didn't know Gcash delivered card statements. Do we get them through emails? Through texts?
Edit: Found this. I guess OP can request a card statement. I just hope this card statement is different from the one we get in the app. That one will NOT give any details other than a reference number. In the end, I guess OP should contact them to get the 4-digit code.
"Upon the GCash Subscriber’s request, GXI shall send a copy of the GCash Account/Transaction History statement either via mail, fax or email, as may be applicable... GXI may charge a fixed amount for Account/ Transaction History Statement fee per request."