I have installed ansible in on of my GCE Debian VM Instance(1). Now I want to connect to another GCE Debian VM instance(2). 
I have generated the public key on Instance 1 and copied the .pub key manually to the the authorized key of instance 2.
But, when I try to do the ssh from 1 to 2 it gives permission denied. 

Is there any other way round? I am a little new to this, trying to learn. 
is there any step by step guide available? and also what is the exact ip address to do ssh on? will it be the internal IP or the External IP taken by GCE when the Instance is started.