A loss is represented as a negative number (a number less than 0) and a gain is represented by a possitive number (a number grather than 0)
For the given data you get the next graph:
The relationship as 0 is not change is: The loss is a change of $800 being a decrease of the amount in the account. The gain is a change of $800 being a increase of the amount in the account.
Both are changes of $800 but represent different types of changes (increase and decrease)