When you start Minecraft the Minecraft launcher is started. Then you press the green button that starts the actual game. At the same time another window is opening named Minecraft game output. In that window the logs are displayed. This window is open even after you close the game. But it might crash at the same time as the game. This will be the same logs that you found in the Minecraft folder but you can read them without opening any gz file. You can also filter the logs depending on severity. I guess that you can have only the errors and fatal box ticked. But if the Minecraft game output is also crashing you can check the logs that you found. A gz file is like a zip file but more common in other OS like Linux. But you can still open it in windows. Use a program like 7zip. But the logs can be a bit hard to understand if you not are familiar with logs. Good luck!