OP's Fault for leaving car unlocked.
Thieves Fault for stealing from the car.
Both are at fault for leaving the car unlocked and for stealing from the car.
You can't be blamed for someone stealing your things. The car door being unlocked is also irrelevant. Opportunist thieves will take what is easy, if they see a gps sitting right there in plain sight, it's gone. Smash and grab leaves no prints and if there are no witnesses or CCTV they get away with it 99% of the time.
its not your fault at all, its the thief's fault he stole from you. you just could have possibly avoided it. but you can't live your whole life like a hyper-vigilant paranoiac.
"Just because you read it on the internet, doesn't mean the person actually said it." - Thomas Jefferson
I leave my passenger door open accidentally sometimes because I've dropped a friend off somewhere. I wouldn't expect them to know to lock the door. It's your unlucky fault that the gps got stolen.