How to sign out of all devices currently using the Apple ID

Changing the password should be sufficient. You can see which devices are currently logged in with your Apple ID by following the instructions provided in this Apple support article: Check your Apple ID device list to see where you're signed in

You can also remove devices from your Apple ID profile by clicking the Remove button that appears in the device information box after you've clicked on one of the devices.

As an added step, I would highly recommend turning on two-factor authentication after you get everything squared away. You can read more about two-factor authentication and how to enable it in the following Apple support article: Two-factor authentication for Apple ID

does Apple automatically sign out all connected devices when changing your password?

If they are still signed in (at the time you change the password) it will not sign them out, but next time they will have to use the new password to sign in.

Your problem is clearly with someone who cracked your old gmail account.

Now they have created NEW apple id and can verify it since they have access to email.

• go to that gmail and change the password to a strong one.

You only got a copy of that email since you have the forwarding turned on.

You must have created a apple id once upon a time since it is needed for basic service like system software updates.

Your original apple id might be still OK and not compromised (but change the password on it anyway).