I have public and private SSL keys generated by puttygen.
I paste public key to GCE console (edit my compute engine instance) in SSH Keys section and I paste there :
ssh-rsa string_of_my_public_key bitnami
string_of_my_public_key is without spaces. Then I added a private key in Filezilla Settings and enabled "debug output" option for SFTP connection and if I try to connect I have debug output in Filezilla:
Offered public key from "path_to_my_private_key.ppk"
Server refused our key
Disconnected: No supported authentication methods available (server sent: publickey)
If I try to connect with putty I have the same message.
I have also another instances in GCE with differents IP's and I can connect with them without problems (instances have their own generated by me SSL keys).
I also tried to generate another pair of SSL keys but it doesn't worked for me.