This is a strange one indeed. There are a lot of questions going through my mind right now that I don't have answers for, primarily because I can't test them out myself.
-If you are having this issue, is it replicable without updating to the latest version (pre version 1.50) of GT Sport?
-Can you access the garage in Arcade Mode? Any mode in Arcade that allows you to select a car from your garage should be good.
-Does the game crash when trying to access certain parts? Or is consistent when performing certain actions?
-What happens when you press the Options button on your controller from the main menu? This should brings you to your profile page. Can you access anything under this menu?
I'm not sure if it's save file corruption (like when a check fails when it receives invalid or garbage data, returning a catch all condition like 'The Garage is Full'), an online synchronization issue (online save), or if there's something else I'm not thinking of.
From what I am seeing, because of this "garage is full" error, the game may not be properly updating the player's garage list, hence why their garages are showing 0 cars or certain lists aren't populating correctly.
Not sure how much help I can be...
1. Delete the user of PSN account that is having issues.
2. If that doesn't work then download the software update file of the console to a USB
3. Lastly if none of those work they said they would warranty the PS4 and I would need to send it in. I wonder if the other guys having this issue also just bought a PS4 or if it's only the game
These steps to me don't seem right for this issue...
1.I'm assuming that this is referring to the local account on your PS4. This really should be a last resort option when all else fails, minus some more extreme steps. This will delete everything associated to the account on the PS4, including all game saves on the console (your game saves on external USB devices and cloud saves won't be affected).
2.This is for a system update, not the game update. You can only perform game updates via PSN. I don't think this is an issue related to the system firmware.
3.This is if you are having issues with the PS4 itself. Seeing how this is a software issue with an application, this is completely unnecessary. From what I've read, people are having the same issues on years old consoles.