How to transfer all contacts from Gmail group to iCloud group on iPhone?
You have two options here - you can either keep the contacts in Google Contacts and add your Google account to the other phone so they sync, or you can transfer the contacts to iCloud. If you're only using Apple devices, my recommendation would be to transfer them to iCloud, because it's simpler to manage and works well with Apple devices. If you own Windows or Android devices, keeping them in Google Contacts will allow you to access them on those devices as well.
Keep contacts in Google Account
To add your Google Account to the other phone:


Open your iPhone or iPad's Settings app.
Tap Accounts &amp; Passwords and then Add Account and then Google.
Enter your email and password.
Tap Next.
Switch &quot;Contacts&quot; on.
At the top, tap Save.
To start syncing, open your iPhone or iPad's Contacts app.


(from Google Support)
Transfer contacts to iCloud


Go to Google Contacts.
Check the box next to any contact and in the top left, click Selection Actions and then All.
In the top left, click More actions and then Export.
Select vCard (for iOS Contacts).
To save your file, click Export.


(from Google Support)


In Contacts on iCloud.com, click the Show Actions Menu pop-up button in the sidebar, then choose Import vCard.
Select a vCard to import.


(from Apple Support)