Change apple ID on iPhone

I have two apple IDs - one is iCloud and the other one is Gmail. On my devices, I'm logged in using the iCloud account but for online purchases from Apple, I had by mistake created a Gmail account. While applying for my Titanium card, I entered my Gmail as the Apple ID and it was processed and approved. But since on my iPhone the account is associated to the iCloud email, the wallet does not show me any option to add my Titanium card. Also since I don't have access to the card, I cannot change the email associated with it. When I try to change the Apple ID on my iPhone Settings, it doesn't allow me to login using Gmail and returns the error that the email is being used by another account. How do I -

• Change the Apple ID on my iPhone so it changes the Wallet properties so it can automatically reflect my Titanium card in the wallet?