The 10 Plat pop up is what we call the "Bane" item. As the Event Information describes, this can help you fight the bosses by protecting you from goblin poison and slowing effects. It is optional and the event is tuned so that the bosses (with a good group) are still doable without Bane Items.

Also, if you log in and play everyday for the event you can get to Gold without spending Platinum for entry into boss fight zones.

Have fun in the Goblin Event!