Answer: Gain of $600,000
Explanation: As we know that :-
Gain / loss = Sales value - Cost of building
Now, we can compute cost of building on date of sale as follows :-
cost = purchase date cost - accumulated depreciation
    = $850,000 - $ 200,000
    = $650,000
putting the values into initial equation we get :-
Gain = $1,250,000 - $650,000
    = $600,000