How did Demikas know the location of the gold room? In Log Horizon, how did Demikas knew about the location of the room where gold is processed and distributed on the Yamato server? 

This happened in Log Horizon II episode 11 during the raid, when Demikas pulled Shiroe away from the raid and threw him into the corridor with the door leading to the gold room.
Demiqas was part of the raid party so he was privy to all of the information that was shared throughout the group. It would make sense that all of the members or the raid know the layout of the dungeon. The Depths of Palm was a raid that was done numerous times before but never to the extent of going all the way to the Gold room guarded by the Kunie Clan.

It is probably safe to say that since Shiroe was able to come to this conclusion by way of research or rumors of the room during the time before the Catastrophe. It is also to note that Shiroe is friends with ReGan, and ReGan has a very extensive library at his disposal. ref(Log Horizon Wikia)