The reaction force to your push is the push of box G against you.
Newton's third law states that:
For every action, there is an equal opposite reaction.
So if an object x pulls or pushes an object y with a certain force, object y also exerts an equal and opposite force on object x.
Now, if you should push box G with a certain force, box G will exert an equal opposite force on you.
Hence, the reaction force to your push is the push of box G against you.
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