Somehow you must be missing a detail, there's no reason for the Calendar app to not see the GMail calendar if the GMail account has been created and Calendar setting is turned on.
In re-reading your posts there are a few fishy things:
1- Your Apple ID has no relation to this. Even if your old GMail address is being used as your Apple ID that will have no effect on your Calendar. The key question is, in Settings do you still have this old e-mail address set up as an account? If so, delete this old GMail account from Settings/Mail if you're not using it. Deleting this account will have no effect on your ability to buy things from the iTunes store. You should only have the GMail account you're using entered into Settings/Mail. Once you do that, make sure that Calendar is turned ON for this account, and make sure that this account is being used as the default Calendar account in Settings/Mail (in the Calendar section).
2- In your first post you said "I have the Gmail app set up as my new email account now" The GMail app is only for mail, it does not provide Calendar support, so if all you did was add the app and sign into your GMail account, that's why your Calendar is not updating. If this is what's going on, you need to add your (new) GMail address as an account in Settings/Mail. If you're sure that you did this, there is no reason for your Calendar app to not see the caldendar associated with this account.
3- In your first post you said "I've linked the two email accounts, shared calendars with both accounts, but it's still not working" I suppose you did this as part of your troubleshooting, but there should be no reason to link two GMail accounts to get your calendars to show up.