Ok, the UD rules say that GC Assailent's Effect TARGETS a monster. So before
the attack goes through, a chain occurs. " I attack your Berserk Gorilla, and Chain
by putting it in defense. Berserk Gorilla's Effect should now activate, thus killing it's
self, now a replay comes right?? These rules apply for Spirit Reaper, because when
it is TARGETED by a card, it is automatically destroyed, so when assailents targets
gorilla to put it in defense, it should automatically be destroyed, thus letting me attack again!
There are A LOT of Gorrilas in my store ( and Soldier's as well...) So this would be a nasty Suprise

From UDE.com




"Gravekeeper’s Assailant" changes Attack Position to Defense Position or vice-versa. It cannot flip a monster face-down because that is not a change of battle position.

You make the change of battle position due to "Gravekeeper’s Assailant" at the beginning of the attack, so you can change the battle position even if the attack is then negated by "Magic Cylinder", etc.

If "Imperial Order" and "Necrovalley" are both in play, "Necrovalley" is still on the field so you can still activate the effect of "Gravekeeper’s Assailant".

The effect of "Gravekeeper's Assailant" targets 1 monster.

If "Gravekeeper’s Assailant" attacks, and activates its effect to change the battle position of the opponent’s "Spirit Reaper", then "Spirit Reaper" is destroyed by its own effect. Then a replay occurs because the potential attack targets on the opponent's side of the field were changed. In this case, "Gravekeeper’s Assailant" can attack again, and can activate its effect again.

If "Necrovalley" is on the field, an attack is declared with "Gravekeeper’s Assailant", and the effect of "Gravekeeper’s Assailant" is activated, and "Mystical Space Typhoon" is chained to the effect to destroy "Necrovalley", the effect of "Gravekeeper’s Assailant" is still applied because "Necrovalley" only had to be on the field to activate the effect.


The same will apply with Berserk Gorilla. You attack, declare the target of the effect, berserk gorilla changes to defense, destroyed once shifted to defense, attacking target lost, replay occurs.