The D-Mat gun was an omnicalculator capable of removing an object from reality.
Physically small, it was a quantum computer linked to the causal nexus much like the Homeworld's timeships. It scanned the target's timeline, and then remodeled the entire universe without the target. The gun "remembered" the victim, but other than that, it was as though the object never existed. If it was a being, it remained aware, but unable to interact with anything or anyone.
The real D-Mat disappeared during the early part of the War. Fake D-Mat guns were often given to troops on forced regen missions, and they were also popular with the Fourth Wave units which had a fondness for burlesque devices.