Some do gym, hoping to get back that’s amazing (2,2,3)

'that's amazing' is the definition.
(oh my god can be an expression of amazement)

'some do gym hoping to get back' is the wordplay.
'some' indicates taking the first letters.
'hoping' becomes 'oho' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to get' means one lot of letters goes inside another.
'back' says the letters should be written backwards.
The initial letter of 'do' is 'd'.
'gym' put into 'oho' is 'ogymho'.
'd'+'ogymho'='dogymho'
'dogymho' in reverse letter order is 'OH MY GOD'.

Can you help me to learn more?