Say, if 5 enemies were killed at once, the condition "enemy hitpoints<=0" will pick 5 enemy instances. But Enemy.rewardGold expression inside this event will only apply to the first instance among those 5.
So yes, in this event you need to add "For each enemy". This will process all 5 enemies, one instance at a time, adding their rewardGold to the global variable.
By the way, this event #40 will run on every tick, which is not good for game logic and performance. I suggest making a function EnemyDeath and calling it when damage is dealt to enemies.