This is a term used in sports and entertainment, including baseball, to refer to the amount of money brought in through ticket sales. It is a broad category that encompasses ticket sales in general, but could also mean tickets sold through the venue only. The phrase is used in baseball and other sports because one used to have to enter the stadium through an actual gate. An admission fee would be charged at the gate, and the total amount collected would be tallied under gate receipts.
Most every modern sports team has many different outlets on which they sell tickets. In fact, most teams do not sell a majority of their tickets in the stadium but rather online. These numbers usually still count as gate receipts, though they may not, depending on the situation. This number is not important to the actual game on the field, and is only used when discussing team business.