Get back = return to someplace.
If I start out from location A, I will say "I will get back by <some time>" = I will return by <sometime>
If I say 'go back', I started out in location A and am currently in location B and I mean that I will return from B and arrive at A. <In China> "I will go back to the U.S. next week.
If I am in location A, or in location B, talking to a person in location A, I will talk about coming back to A.
"I'll come back to the office tomorrow." = I will arrive back in the office tomorrow.
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