You have the first GK's, 03 & 04. Bubbles on the headlight clear cover.
You have the GK FL (facelift), 05 & 06. Same size headlights, but a nice smooth curve on the headlight clear cover.
You can can just swap headlights between GK & GK FL's, simple bolt in although there may be a minor wiring change.
Then you have the GK FL2. Different design headlight which also means different hood, front fenders and bumper cover. There are also other body changes.
95% of parts fit any year except some body parts.
There should be a label on the open edge of the drivers door with the month and year build date. In the US, there is also an emissions label under the hood stating what year standards it was built to.
This site also has a VIN decoder to tell you more as the car was built.