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.
I struggled with this as well. I uninstalled Google Messages and then got the option to install Samsung Messages. Be sure Samsung Messages is your default on the phone.
It makes a huge difference because Bixby will not work with texts from the watch if your default is Google Messages. Samsung didn't think that one through. ๐
