The default SMS app is the Samsung app. Settings/apps - find the app, More and you should be able to make it not the default app - reset defaults or something. (I've been saying, for years [maybe there's a time delay between when I post and when Google sees it, because I was asking for years for the a/b system before they finally implemented it] that "reset defaults" should be renamed to "stop using this app as the default [in this case] SMS app.)
Next time an SMS comes in, you should be asked to choose an SMS app - choose GoSMS.