Griffon Heavy Mortar Carrier
At first glance, the Griffon seems to fall in a strange "in-between" zone -- the heavy mortar is too large to be man-portable, but it isn't large enough to achieve the same range or firepower as its larger cousins. Mounting it in a Chimera chassis almost seems a waste of both, so many regiments that are not specifically dedicated to artillery don't bother replacing Griffons as they are lost (and wish they had once they hit the next fight).
However, it is precisely the relatively light weight of the heavy mortar that gives the Griffon certain advantages over the other artillery pieces. Because the shells are smaller, the cannon is easier to reload and can achieve higher rates of fire. Because the Chimera is so much heavier than its cannon, the recoil is not as severe, allowing the Griffon to precisely drop shells on target. While it lacks the destructive potential to crack open a fortress or destroy a siege tank, it is more than a match for infantry and light vehicles, and so it can focus on destroying the enemies lighter, more mobile elements and leave the Earthshakers free to engage the harder targets.
- The default (and so far only known) pattern of Griffon doesn't have a tall, curved gunshield like the Basilisk or Medusa. Even though Vanaheim also makes angular variant gunshields for those other artillery pieces, the Griffon gunshield is much lower and more angled inward than the others.
The Griffon's heavy mortar is not much to write home about (Range 12-48" S6 AP4 Ordnance Barrage 1 Large Blast), but when it is mounted in the Chimera (rather than on a static platform), it has the "Accurate Bombardment" rule, which allows it to re-roll the scatter dice. Unfortunately, the increased rate of fire and portability alluded to in the vehicle's lore does not come through in the rules.
6th Edition - The Griffon has officially been removed from the glorious* Astra Militarum Codex. From now on to use it you'll have to use the Forgeworld Rules which are in Imperial Armour Volume One: Second Edition
Or proxy it as the new Wyvern, its better replacement.