Found a gong in region 2

[Setorrs The Lollygagger]

Read that this is some side quest i can start. Does this tie in with the mask areas i see on the map? How many gongs are per region, i know the total is 5?

[Cyiel]

There is one mask per region. Each one will give you a passive. When you have all of them you will have access to the secret boss located in area 2.

[Melee Only]

Hey, I just finished that exact side quest
Yes it is connected to the mask but I won't spoil you how
The idea is that for each hit on the gong you summon 3 of those flying creatures (there's some max limit). If you hit one of them mid-air your jump regenerates which means that you can use it to climb that cliff. However you need to have the region 2 ability unlocked which allows you to dash through the gravity canals and you shouldnt definitely purchase the upgrade that allows your melee attacks to destroy projectiles first!
Regarding the amount per region I can't help you since I haven't finished the game yet, maybe someone else can