I got locked out of my Gmail account because my iPhone didn't save its suggested password

At first, you talked about keychain then you are referring to your Apple ID. There is something amiss here.

Are you referring to Gmail as Apple ID

then...

If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support

If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support

else...

Change your password · Open your Google Account. You might need to sign in. · Under "Security," select Signing in to Google. · Choose Password. You might need to ...

Change or reset your password - Computer - Gmail Help

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.