Hey JesseLoveday,
We'd be happy to help out. While this may be a better question for Google Support, we want to make sure you have all you need to get a solution.
To get started, here are the steps to add your Gmail account automatically on iPhone 12:
How to set up your email account automatically
1. If you use an email provider like iCloud, Google, or Yahoo, Mail can automatically set up your email account with just your email address and password. Here's how:
2. Go to Settings > Mail, then tap Accounts.

3. Tap Add Account, then select your email provider.
4. Enter your email address and password.
5. If you see Next, tap Next and wait for Mail to verify your account.
6. If you see Save, tap Save.
This is outlined further here: Add an email account to your iPhone, iPad, or iPod touch
If this doesn't work for you though, one thing we notice is that you're still running iOS 15 on your iPhone 12 which is compatible with the latest software which is iOS 16.2. To isolate the issue further, we'd suggest updating your iPhone and testing the above steps again to see if the issue persists. For help doing so, check out: Update your iPhone or iPad
If all else fails, here's a link to Gmail support: Gmail Help
We hope this helps out.
Take care!