Is there a safe way to autologin a user without requiring to enter credential every time?
If you are implementing GameSparks with Unity when you set up a new project with the Unity SDK and perform an authentication request, an authentication token is stored locally. If you then start your scene again and do not call an authentication request, one is still performed in the background, authenticating the user and using the stored authentication token.
You can find more about how to set this up here.
The Unity Auth Token will not expire once it has been successfully obtained, should you wish to force a reset you could use this SparkPlayer call.