When you define a field for your mobile wallet pass in Wallet Pass Definition, one of the required values is a Key for the field.

When you define a pass just to use on Apple device with Apple Wallet, that field can be anything you want. 

For use with Google Pay passes on Android and Apple devices, you need to map that key to a proper field.

Follow this links for a property mapping:

As in illustrated below, in order properly map event name, you need to use the key expected by Google Pay, which is class.eventName. You can see the proper key mappings from the link above.