Note: This page is taken from n.wikia.com.
(As written in the N in-game help)
The bad news: these guys fire depleted uranium at super-sonic speeds. The good news: they can't move, and their targetting software is obsolete.The gauss turret is an effective enemy. Just like the homing missile turret, it doesn't move. However, it has a tracking device that is visible in the form of a crosshair. The crosshair homes onto the ninja when it's in its sight (in a manner same as the homing missile turret), and fires when the player is in or near the crosshair. One can tell by the way the crosshair changes into a red colour, which means it will fire immediately. It's a good idea to get out of the way, since it fires depleted uranium that crosses distance instantly and kills the ninja upon contact.
Multiples of these are absolute hell. Multiples of these in a difficult terrain is even worse. Oftentimes, a single gauss turret is placed to overlook an entire area, making the area completely unsafe and difficult to traverse. And in 1.4's 29-0: crossfire, the player may find it hard to keep track of multiple turrets at completely different positions, and with different crosshairs. It is easy to keep track of four turrets near each other, but four spread out over a large area where there is nowhere to hide is much more difficult. When these show up, one needs to be on one's toes, as they are always a threat.
To avoid a gauss turret, it is recommended to keep moving. Fast. And not stop moving. One should not run directly at the turret, unless there's a chance of moving out of the way at some point. It tends to fire a bit in front of the ninja to catch him on the move, so if the player needs to, they may stand still when the crosshair turns red while it doesn't seem to be directly at them. And one has to be wary of a gauss turret shooting from below them while they are walljumping. The moment they reach a corner, it fires, making this a very difficult situation indeed.