I keep getting text messages that my message verification code is g- followed by numbers

It’s not Apple ID; it doesn’t have a letter prefix. The G- I think means its Google or gmail. But if you haven’t done anything that would require authentication someone is trying to hack your account, so don’t respond to it. And change your password, because you won’t get an authentication request unless the user ID and password are valid.

As Lawerence Finch has mentioned, this is most likely an authentication request from Google. If you attempted to access your Google account from your iPhone, AND you enable two-factor authentication on that account, you would receive these types of texts ... otherwise, as mentioned this may be a means that a bad actor is attempting to gain access to your Google account.