Ahem.
The fourth Lightspeed Gravity would hit well before the third non-lightspeed Gravity.
You're saying "3 gcds" and "4 gcds" while ignoring the fundamental flaw in this discussion - Gravity has a 3s cast time compared to the 2.5s gcd. With lightspeed, each cast of Gravity takes 2.5s to refresh, without it, each Gravity takes 3s to cast.
If we assume you start both of these at the exact same time, here's what we're looking at:
So let's not pretend like this is a loss, when it absolutely is not.
To say nothing of the fact that these 4 Lightspeed Gravities that hit faster than the 3 hard-cast Gravities both deal the same amount of total damage and cost the same amount of total Mana. You can shave 1s off your total DPS cast time for the same damage, while still having Lightspeed ready to drop cleric and spam heals on your Tank after it stops.
It would be theoretically more effective to do G1>G2>swiftcast>G3, so THAT would beat out the Lightspeed approach. Let's just not poop out false/bad math pls.