As the title of this post says, does the Demonic Gateway drop threat on the Cosmic Sparks? If it does, for how long? The debuff of 15 seconds says "Faded into the Nether causing most creatures to prefer other targets." But is that how long you lose threat and can use the gateway again, or is that just how long till you can use the gateway again?
As long as you have the 15s gateway debuff, your aggro towards the sparks will be 0. But all the threat you generate during the debuff will be back when it fades (You can then use the gateway again).
The gateway does not reset your aggro, it just temporarily sets it to zero.