On the watch it really doesn't matter which messaging app you use, your messages will still come to the watch as long as you have it set to allow the notifications. For instance, if you use Samsung messages on your phone and allow the notifications to come to the watch, you will be able to read and respond from Google messages. You can even still use Samsung Messages on your phone, download Google messages and not use it, but allow the notifications from Google messages to come to the watch. Caution, though, if you allow both you will get duplicate notifications.
