No, Darkrai is not able to learn Earthquake.
Darkrai is the only one who can learn it by level, although Smeargle can use Sketch to learn it as well.
Yes, Darkrai can learn the moves Roar of Time and Spacial Rend.
Darkrai
In the Pokemon games Darkrai learns the move dark void at level 66. He is the only Pokemon which can learn this move.
Darkrai
Yes, Gallade is able to learn Earthquake through the use of the Earthquake TM.
Gastrodon does not learn Earthquake naturally, but it can learn it through use of a TM.
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Yes, it can learn it by using the TM Earthquake
Nidoking can only learn Earthquake when you use TM26 on it.
Aggron can learn earthquake by using the TM on it.
metang doesnt learn earthquake.