Hi,
The process should be:-
Open Messages.
Click the + icon under the list of iMessages and Bonjour.
Select the Google Option
Use the ID as it appears when you login into Google (Google have two forms of ID @gmail.com and @googlemail.com) and Messages is more fussy than the Mail Login about which you use.
Use the Password you use at Google
You may, at this point in Sierra, get the message that it is already set up.
You may have seen it check the ID against the gmail and googlemail database (Google treat them as the same, it was originally used to denote if you were in the United States or not)
IF this is the case and you do use it for Mail then go to System Preferences > Internet Accounts and then the Google listing and Enable Messages.
Once this is done you may need to check the Server Settings tab on the Google Account in Messages and check it is using talk.google.com as the server and SSL with port 5223
I recently had trouble with this and had to remove my Google account in Internet accounts and then start in Messages to set up the Google account (which is added to Internet Accounts) and then enable Mail in Internet accounts afterwards to make sure the Messages app connected to the right server.
You will also have to visit the Google web site and go to the account's Settings and the security settings and Allow "Less Secure apps".
There is debate as to whether this is less secure or not but Google have deemed SSL connections as Less Secure. SO for apps that use SSL it has to be set.
Strangely they still allow Mail to connect using SSL without needing this setting.
8:19 pm Wednesday; January 18, 2017
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 10.x and an iPad (2)