I've just finished both arc of the story (Capsule and Secret), but I am confused. Does the manga plan to continue the series under Gacha Gacha name with another arc/setting/cast? I asked this because there are multiple sources that said Gacha Gacha series is not yet complete.

I believe that the series has ended, though I'm not sure...

Riches take to themselves wings and fly
Power can depart suddenly from the powerful
Pride goes before a fall
It's an ill wind that blows nobody good
There is no fool like an old fool
Insolence attracts the thunderbolt of the gods

In other words, don't get rich, powerful, proud, old, or insolent, or else you'll lose everything, be struck by lightning, and blown off a cliff (by a proud, diseased wind!).

The Gacha Gacha series was completed this June in the year of 2008. Shame too, I really enjoyed the "Capsule" story.

The series is totally complete unless the author decides in the future to write more.

Capsule takes up volumes 1-5 while Secret takes up volumes 6-16. They are not related in terms of characters or plot. Your "sources" that say this series is not complete are wrong

A just ruler amongst tyrants