The gross win of a gambling business is the amount it has won (and its customers have lost) over a given period.
As a high proportion of the total amount paid by customers as stakes is returned to them (and never becomes the bookmaker's or casino operator's money as such) the amount of gross win is an important measure of how much money is coming in. Investors should also look at the total amount staked. The latter is a better indicator of the size of an operation and its market share.
The gross win can fluctuate randomly as a result of wins and losses. In case of casinos even a single individual may have a significant effect. Bookmakers can also be badly hit by a few wrong outcomes: events like a string of favourites winning races would be quite damaging to a bookmaker. This can make underlying trends harder to discern.