Unable to remove suggested recipient in Gmail

If you are using the Mail app to access your GMail:

The suggested recipient would likely be suggested from (info. in the recipients card in) the Contacts app.

You can edit or delete Contact cards if you want.

Apple support's article: Add and use contact information on iPhone - Apple Support

Eg: says you can:

Delete a contact

1. Go to the Contacts app  on your iPhone.
2. Go to the contact’s card, then tap Edit.
3. Scroll down, then tap Delete Contact.

or

If needed: See how to Edit contacts on iPhone - Apple Support

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If you are not using the Mail app : You may need to log-in to GMail online to remove that recipient ?