The basic role of the gunner in a squad is to attract fire. The gunner lays down a large volume of fire, and becomes the key target. This allows the other members of the squad to return fire and nuetralize the threats. In a game, the gunner will always attract attention. It is the loudest and most sustained weapon available. If you learn to use it well, you can take out opponents at ranges of up to 800 yards. The classic machine gun fire creates a "shotgun" pattern, where the gun creates a natural spread due to the recoil and movement. As the bullets travel further out, they create a flatter pattern, with a greater spread. If the game creates a realistic pattern, you can use this to your advantage. The machine gun is the best weapon for punching a hole in the opponents lines. A profecient gunner can take out multiple targets before going down.
The gunner is usually not the best choice for a point man during movement. The gunner is slower, and easier to hit. If a gunner forms a team, he should be following, and as soon as someone engages the point man, the gunner is able to open up and lay down maximum covering fire. This draws the opponents attention, and allows the point man to go for cover or scores, depending on situation.