Tire wear is VERY dependent on the tire itself. GP5000s can be replaced when threads are showing as others have said but really if its that long, and you are down tot the puncture belt but I doubt everyone would want to wait that long.
I run durano plus tires for training, you can ride them down to the blue puncture strip with no issues however you better replace these at this point since the tire was not designed for use like that.. and you look stupid..
Schwalbe DP also develop that flattened section (I've never seen this with GP5k actually) as OP says, some people replace them at this point but I like to get more out fo my tires than that. Unless it affects the cornering, you will be ok.
Also front tire wears at about 60% rate of rear, so you can swap them after halfway the expected replacement interval for extra use if you are really concerned