Summons to Lord Greymane

Simply navigate through your Adventure Guide and start Gilneas Reclamation. If you want to get the quest from a npc, then:
The questline starts for [faction=469] characters with a Greyguard Elite, right next to the principal Auction House in Stormwind or next to the Stormwind portal in Valdrakken. Talk with him to start the quest Summons to Lord Greymane, asking you to meet up with Genn Greymane in Stormwind Harbor.

In the quest To Gilneas, Greymane announces it is time to take back Gilneas, but the Scarlet Crusade has moved to the lost city to use it as a stronghold. As such, we're moving in to kicking them out. While we fly to Gilneas with Genn and Mia Greymane, Greymane talks about the Scarlet Crusade - Specifically, about how he has little experience with them, as they rose to power after Gilneas close themselves off to the rest of humankind.

Genn Greymane says: Gilneas will be ours again. Let me know when you are ready.
Mia Greymane says: Thank you for offering your aid, <Player>. I knew you'd come.
Genn Greymane says: I have little experience with the Scarlet Crusade. They rose to power after we walled off Gilneas.
Genn Greymane says: My understanding is that they were originally formed to eradicate the Scourge. A noble enough cause.
Genn Greymane says: But their zealotry has caused them to turn on any who did not join their crusade.

[faction=67] characters start the questline with a Deathguard Elite, right next to the Pathfinder's Den entrance (Portals hallway inside the Gates of Orgrimmar) or next to the Orgrimmar portal in Valdrakken. Talk with him to start the quest Clandestine Movements, and you'll be requested to meet with Calia Menethil and Lilian Voss at Pyrewood Village in Silverpine Forest.

In the quest To Gilneas, we learn that Lilian has been in contact with Tess Greymane due to their connection via the Uncrowned, and using that connection to return Gilneas to the Alliance - but the Scarlet Crusade has taken residence in there. As they're a common enemy between the Horde and the Alliance, Lilian is more than happy to help with killing them off.

Lilian Voss says: I have an Alliance contact meeting us by our boat. We are going ahead of the army to offer our raid.
Calia Menethil says: And to ensure we don't take King Greymane by surprise. His dislike of the Forsaken is well known.
Calia Menethil says: Watch yourself, tensions will be high.
---
Lilian Voss says: Tess, what can we expect?
Princess Tess Greymane says: I should ask you--you're the expect. Gilneans know precious little of the Scarlet Crusade.
Lilian Voss says: At one point, we--they fought against the Source. But fanaticism eventually blinded them.
Calia Menethil says: Now they view everyone as monsters to be cleansed by the Light. Eradicated.

After completing Beginning a New Dawn you'll get The Wall Between Us or What We Left Behind depending on your faction.
Besides of the [item=211873] mount, [spell=430283] transmog, Reclamation of Gilneas achievement, and [item=211788] toy are granted aswel.
It doesn't matter race or faction, log out then come back online after finishing the entire questline. You'll have the toy in your Toy Box, but is usable for Worgens only.

If you are on The Wall Between Us, Lilian and Queen Calia in Pyrewood Village are having a conversation.

Calia Menethil says: Thank you for your assistance, Funkowah. I know I asked much of you in aiding the Gilneans.
Calia Menethil says: I only hope that Tess continues to prove more... diplomatic than her father.
Lilian Voss says: I know her well. She cares far more about what people do than what they are.
Lilian Voss says: Still, it's probably best if we give the Gilneans some space for the time being. Let them enjoy their homecoming.
Calia Menethil says: As they allowed the Forsaken to do, not too long ago. We've kept our word. It's time we returned home as well.

If you are on What We Left Behind, Greymane asks for some time, as he needs to visit somewhere, and Tess asks us to check on him in What We Left Behind. Turns out, Greymane is visiting Liam Greymane's tomb to talk to his son and his regrets on building the wall.

Genn Greymane says: I don't know if you would recognize me, Liam.
Genn Greymane says: All these years, the lessons I have learned. The Alliance has always offered an outstretched hand.
Genn Greymane says: There are days it feels right, and days I wonder... what if I had not built that wall?
Genn Greymane says: Would Lordaeron still stand? Would you still be...
Genn Greymane says: Ah, but listen to me. The ruminations of an old man.
Genn Greymane says: You did not live long enough to regret as I do.
Genn Greymane says: Your sister will lead the kingdom, now.
Genn Greymane says: No one could be more proud of her than your mother and I. Except perhaps you.
Genn Greymane says: Rest well, my son. Gilneas is in the best of hands.

Link to the entire Reclamation of Gilneas Questline in Patch 10.2.5

But why? :( Why piece of lore is only for max lvls toons instead of being scaled to at least lvl 60 chars, who can also access Valdrakken is beyond my comprehention.

Starts in valdrakken, next to the portals. A greyfang elite gives the quest.