yes figure of speech neither mean anything. used for shocking times.

Yes it is the same, but only older people say Good Heavens when they are very surprised about something. Younger people say Oh- my - God [ OMG] very slowly to indicate amazement or surprise.

Yes, they can both be used in the same situation, but "Good heavens!" is softer than "Oh my God!". It's all to do with "using the Lord's name in vain".

my goodness :P