Gauss Cannon has the most range of any ballistic weapon, and one of only two kinetics with 1000 range or greater. Gauss Cannon is much more damaging than HVD. Heavy Mauler is the only HE weapon with 1000 range, and the rest have less range. Many beam weapons have 1000 range.
Gauss Cannon is great on a Conquest. Conquest is squishy for a capital, and cannot stand up to a enemy battleship like an Onslaught. What a Conquest can do is put long range weapons on all slots, and use its speed (Conquest has Maneuvering Jets), to play keep-away from many enemies and snipe at them. With unlimited ammo, Conquest has become a vicious sniper that can kite from anything that can outgun it.
Dominator and Onslaught are better off just killing everything with Storm Needler or heavy HE.
For all kinetic or all HE, some ships do not have better alternatives. Some ships have too few mounts to use both, or only one type has enough range to outrange the enemy. For example, Hound. Using both mounts for attack not only leaves no PD, both also overloads flux capacity quickly. With unlimited ammo, slowly plinking at shields with HE is a viable option, and its small mount can be a Vulcan to stop missiles and to finish off vulnerable ships. Also, Hound can stay further back while other friends kill shields for them. Also, Heron. Its best long range option, if not using Heavy Blaster, is Heavy Mauler. Why not use HVD? Its fire rate is slow enough that the enemy can lower shields, dissipate (not vent) some flux, then raise shields again before HVD can be fired again.