Im unable to complete the Serendia knowlege 'Eastern Gateway' The tooltip says  "Complete Quest 'To the Eastern Border' but I have no idea how to start this quest chain... Ive completed all quests in Heidel and completed the Imp quest chain... my Amity with Cruhorn Wyrmsbane is 100% .. 

Lvl 56 - Completed Southwest Calpheon and am questing in South Mediah

Im at a loss on what to do, any help appreciated. Thanks in advance

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Same question...

Also having the same problem. Have no idea how to get the quest started. I really want to complete my topography. level 57 questing in South Mediah, not sure how I missed it

Im at lvl 57 and stuck with the quest too. I've created an alternate hero(same server and class) to clear that quest but it didn't obtain knowledge too.

The quest is a part of "An Interesting Rumor" story. You can access the story after you complete all the main quests.

I'm stuck on this too! Did anyone figure it out?

I am a little late to this, but I just stumbled upon this too. 

To do the quest line, open the menu, select Story, select "An Interesting Rumor" side story. One of the first quests they have you in that storyline is "To the Eastern Border Gateway".