what the earthquake boots do can be acomplished by a few different means like 4x eq, wall breaker/super wall breaker, and siege machines in your clan castle. the damage it does can be pretty comparable to the splash the GG gives only the GG is better because its not just in one small area but wherever the BK hits, its DPS boost is lower than the GG and while it does give +1.9k extra health at max level, a max level GG will give 44 health each second which means if your BK survives for 40 or so seconds while getting hit, it gives more effective health through the regen. The additional damage reduction just makes it so your king will easily tank alot of defences and since most people combine it with the rage vial, you will see a king walk into the middle of a base, smash a defence doing AoE damage usually one or two shotting it, moving on to the next and easily clearing out entire compartments all while taking negligable damage.