Was the MCH level 53 or 54/synced? Gauss Barrel on its own is usually a net loss because it also stops auto-attacks (which are actually a fair bit of DPS); the only exception to this being when spamming AoE, the bonus AoE damage outweighs the loss of single-target AA damage.
Gauss Barrel really only becomes useful later on down the line, once you have the skills that can only be used with it; same deal with Wanderer's Minuet. If the MCH was level 53, then them not using Gauss is fine (afaik).