I have a game timer when changed to 0 it ends the game but I was wondering since even if the exploiter changed its value to 0 it still won't change and I was wondering if it would be safe
You are correct.
The only changes that will be replicated from a client to the server/to other clients are those made to their own character/humanoid.
An object in ReplicatedStorage can only be modified globally by the server. Any changes made by clients will only be visible to those clients, and not to others.
If I am not mistaken, RE is a place that can be accessed by the client and the server, therefore if you don't want the client to access the gameTimer and change the value, I recommend ServerStorage.
Somebody correct me if I am wrong.