There is no such place where keeping your wallet private key will be completely safe and secure. Because, where you are afraid of Gmail being hacked after putting your private keys in Gmail. Just like that if I say that keep the private keys of your wallet in a secret place on the mobile. Then you must think that if the mobile is lost?
Then your private key ones will be lost. These words are true in real life as well.
If I tell you that you should write down your private keys in real life somewhere safe.
And if you think that later you forget about the place?Even then you lost your private key ones from there too.
So by no means get that much protection where you can be sure.
If you want to keep private key in place, keep the place safe.
You must save your mobile to prevent your mobile from being lost.
 If you write somewhere in real life, you must remember that place.