How to reach Galskap mountain on issterang
 

How do i get there?
on the map i cant sail to there
im on the mission to close the dark portal

By this point you should have gotten the flying horse, so you simply fly up the mountain.

No i havnt got flying horse
 

In the map i just cant sail there

You mean you can't sail to Isterreng ?

Then you don't have map for Isterreng yet, which can be found on top of snowy mountain in Vestlig Isle, where you found the Valkyrie.

If you ARE already on Isterreng but can't reach top of the mountain there's a CAVE that goes up, infront of the cave you meet another Valkyrie and only after talking to her does the cave open up.