The Gate itself is a pair of massive buildings set side by side, with two structures attached to the inner sides creating a very narrow passage way between. It is designed in such a way that the only way past is through this channel, and forcing large numbers of soldiers through at one time would be impossible. The compound, on the other hand, can easily store large numbers, however sneaking up on it from the other side can only be done by circumventing a portion of the hotel, and even beyond that, getting in without detection would be an unsafe maneuver to attempt.
When Walker, Adams, and Lugo make their way through the Silano Melvini, they come up upon the entrance to the Gate from above. Adams quickly kills the guards, and they sneak up on the overlook. It is clear that there are a large number of people there, however Delta mistakes them for 33rd soldiers rather than civilians. Adams and Walker decide to use the nearby mortar to rain white phosphorus down on the compound, against Lugo's wishes. They fire down until everyone is dead or incapacitated. After walking through the compound and attempting to ignore the screams and moans of those they have killed, they meet a dying man who tells them that they were helping, and Delta realizes their error, having murdered 47 innocent people.
From here on out, Walker's grief turns him to a more primal and aggressive soldier.