You can move notes from Gmail back to your iPhone from an iMac via iCloud.
1) "Add Account" for both iCloud and Gmail to your iMac's Notes App.
Choose "Add Account" from the Notes dropdown menu on your iMac. (Or, add both Mail & iCloud accounts from the Mail app on your iMac.)
Make sure iCloud is added; if not, enter your Apple ID and add. Then, make sure that "Notes" is checked.
Click "Add Account" once again from the dropdown, choose Google, and enter your Gmail account info. Make sure that "Notes" is checked.
2) Open the Notes App on your iMac, and transfer notes from Gmail to iCloud.
You should now see both accounts on the left. Click Gmail. Select all (hold down shift, click first one, scroll down, click last one). Drag all notes to iCloud at left. Easy peasy!
3) On your iPhone, Click "Settings" and choose "Mail, Contacts, Calendars." Under iCloud, make sure that "Notes" is checked ON. When you open Notes on your iPhone, you should now see iCloud full of notes from your Gmail.
NOTE: When you next access Gmail, all of those (possibly unwanted) notes are still there -- even if you can't see them -- in your account's archive. To remove, you'll have to manually delete from "All Mail," then "Delete Forever" if desired from Trash.