My child just got an iPhone SE and will have her siblings send her a text from their iMessage texting app to hers. Instead of her receiving it as a text message, it gets sent to her Gmail account. Her phone is set up to send and receive iMessage from her phone number and apple ID iCloud account email but not her Gmail account.
iPhone SE, iOS 10.2
Disable the Gmail account or Apple ID email (are they the same?) for receiving iMessages:
Settings/Messages/Send & Receive/(You Can Be Reached By iMessage At)
Select only the phone number or uncheck any unwanted email accounts listed.
See If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
Disable the Gmail account or Apple ID email (are they the same?) for receiving iMessages:
Settings/Messages/Send & Receive/(You Can Be Reached By iMessage At)
Select only the phone number or uncheck any unwanted email accounts listed.
See If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
They are separate email accounts. The Gmail account isn't listed as one of the methods of iMessage contact, the text messages just somehow get sent to it.
iMessage being sent to Gmail account
