I ran into this before. Open your Contacts app, and tap your profile pic at the upper right. What's at the very top -- "Contacts from all accounts" or your profile pic with your Gmail account? If "Contacts from all accounts" is at the top, then tap your Gmail account (which will subsequently make your Gmail account appear at the very top). Now tap the hamburger menu icon at the upper left -- you should now see all of your Labels. Tap one of them, then tap the menu button at the upper right, then Send Email.