GCD is 2s, Gauss Barrel cast time is also 2s. So 4s are used, one shot base (for example) 400 damage. GB gives +30% (30% of 400 = 120) total shot value 520.
Comparatively, 2 shots in 4s without GB. Each shot base 400 damage. 800 damage total.
How is maintaining a 100% uptime a DPS gain? Just due to the fact that our Split shot is such a minuscule percentage of our total damage (assuming random proc doesn't screw us)