iMessage not working with Gmail

You cannot send an iMessage directly to a Gmail email address, because iMessage requires an Apple Account or a phone number. 

If your texts are incorrectly sending or receiving through your Gmail address instead of your phone number, you can fix this in your device settings. 

Fix iMessage Using Your Phone Number

1. Open the Settings app on your iPhone.
2. Tap Messages.
3. Tap Send & Receive.
4. Under You Can Receive Messages To and Reply From and Start New Conversations From, check your phone number and uncheck your Gmail address.