There appears to be little symbolism attached to the use of a gun carriage to carry the casket in a funeral procession. The gun carriage may have been adopted as a dignified and suitably military way to save soldiers from having to carry the casket. In the mid-1800s, the Queen's Regulations were amended to permit the use of a gun carriage and horses to carry a casket to a burial ground, but only when it was over a mile away. Since those days, the gun carriage has become an accepted part of military and state funerals.

AWM H12222
The coffin of the Unknown Warrior is transported on a gun carriage past the cenotaph in London, Armistice Day 1920.
AWM H12222