You should switch out the matrix cube with the Dread Guard Relic of Dark Radiance (internal proc damage relic).
The topic of which relics each class should use is discussed in more detail in this thread if you'd like to read more into it. But for Gunslingers, the best relics to use are:
1. Dread Guard Relic of Dark Radiance
2. Dread Guard Relic of Boundless Ages