How to change Gmail notification sound
I find solution that works for me and I want to share it with you.

My gmail notification sound has to even wake me up so I need my custom sound that Gmail app doesn't have.

However iOS Mail app allow you to change notification even with your custom sound - how to make your own notification you can find everywhere on the internet. 

When you use Gmail in iOS Mail you will find out that you can't get push notification with Gmail unless you have paid plan with Google.

If you want custom notification sound of your Gmail with push notification you have to do this:


  
  Make email account which iOS Mail can provide with Push notification (iCloud for instance).
  Forward your Gmail to your new email with push (iCloud)
  Change the notification sound in Settings > Notifications > Mail
  You can change the Alert style on None and you can use Gmail app as you did before only with sound notifications from different app.
  


It's quite easy and it works for me just fine. I do know however, that it won't be ideal solution for everyone.