Yes, since 3.2.x or 3.3.2, when L-Gates don't open too soon, Gray Tempest wreaks havoc in the galaxy, as intended indeed, but AI advanced empires manage to fight back.
Once player can get battleships to complement the Fleet (full BB fleets are not required), then parking into Egress Terminal allows to control the invasion (in my case 2x 20K fleets). I don't know if it's due to a bug, but usually (now) 3x40K Gray Tempest reinforcements will split in one fleet flying by gravity well before heading to the starbase, and two fleets directly heading to L-Gate, one of them sometimes returning after a few months. This allows player to destroy these fleets one to one, and let at least one of them bugging the other empires. Due to a various combat "bugs", Gray Tempest capital ships will spend their first rays of death on corvettes, allowing your cruisers and few battleships to stay alive. Also, nanite strikecraft will target your flak escort destroyers, letting other ships to kill their interdictors. Losses are minimal, and nothing that you can't replace between two reinforcement waves.
These two 20K fleets are also enough to clear one or two systems between two reinforcements. When more battleships are available (if you don't like to win with heavy losses), then you can end the menace at will.
Usually, once it's no more useful to keep letting Gray Tempests enter the galaxy, that is after you could secure most of those territories around L-Gates, should this be your agenda, and you have not enough fleets to repel one Great Tempest at each controlled gate, then it's time to end the menace. And don't forget to close borders to any empire sending a (constructor) ship into L-Cluster, all the time you survey and claim those systems.
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