What is Majestic Myriarch's power and toughness if I "eternalize" it with God-Pharaoh's Gift?
It will be a 4/4.

As the rulings indicate, God Pharaoh's Gift's effect is a copy effect with modifications; in particular, it modifies the type, color, power, and toughness of the copy. this means that rule 706.9d applies:


  When applying a copy effect that doesn’t copy a certain characteristic, retains an original value for a certain characteristic, or modifies the final value of a certain characteristic, any characteristic-defining ability (see rule 604.3) of the object being copied that defines that characteristic is not copied. If that characteristic is color, any color indicator (see rule 204) of that object is also not copied.


This means that Majestic Myriarch's first ability is not copied, so it is just a 4/4.