SHELL

Permission denied (publickey,gssapi-keyex,gssapi-with-mic).

Solved it by amending the 

/etc/ssh/sshd_config 

PasswordAuthentication yes 

then re-started the service using 

sudo systemctl restart sshd

permission denied (publickey gssapi-keyex gssapi-with-mic) ec2

This issue usually occurs when you are trying to ssh outside of the folder you downloaded your .pem files on.
Therefore in order to resolve this you have to make sure that on your terminal, 
you first "cd" to the directory where your .pem files are located, then verify their existence there 
by using the ls command, if they are there you can then proceed to run the  ssh -i XXXX XXXXXXX command.

Hopefully this answers your question.

Thanks.

Permission denied (publickey,gssapi-keyex,gssapi-with-mic).

fixing EC2 instance

ec2 Permission denied (publickey,gssapi-keyex,gssapi-with-mic)

For AWS EC2 instance with centos Linux make sure you're using the righr User