UPDATE 2:
Error I've got from the mapserv.bat:
[Error]: npc_enable: Attempted to hide a non-existing NPC 'dup_1_110211390' (flag=0).
[Debug]: Source (NPC): Varmundt#0 at 1@gl_kh (149,41)
[Error]: npc_enable: Attempted to show a non-existing NPC 'dup_1_110211413' (flag=1).
[Debug]: Source (NPC): Varmundt#0 at 1@gl_kh (149,41)
[Error]: npc_enable: Attempted to show a non-existing NPC 'dup_1_110211414' (flag=1).
[Debug]: Source (NPC): Varmundt#0 at 1@gl_kh (149,41)
[Error]: npc_enable: Attempted to hide a non-existing NPC 'dup_1_110211413' (flag=0).
[Debug]: Source (NPC): Heinrich#1 at 1@gl_kh (149,100)
[Error]: npc_enable: Attempted to show a non-existing NPC 'dup_1_110211415' (flag=1).
[Debug]: Source (NPC): Heinrich#1 at 1@gl_kh (149,100)
[Error]: npc_enable: Attempted to show a non-existing NPC 'dup_1_110211417' (flag=1).
[Debug]: Source (NPC): Heinrich#1 at 1@gl_kh (149,100)
[Debug]: NPCEvent 'dup_1_110211418::OnEnable' not found! (source: Heinrich#1)
** Renaming the duplicated NPCs names scripts/instance npcnames in the OldGlastHeimHard.txt fixed the issues and it's now working well.
Credits:
Thanks again to this pull request #4219 by Questune09 for the NPC Sellers
Mob and Skill database got from this REQUEST. Replaced the NORMAL mode mobs into HARD mode.
Edited the quests IDs inside quest.db (There are questids for OGHH already written inside)
Changed the map instance locations from 1@gl_k / 2@gl_k to 1@gl_kh / 2@gl_kh
Euphy, Ziu, Heris, exneval, Secretdataz, Aleos for the script of OGH Instance.
rAthena community. (Browsing for scripts in the forums and learning it from other people lol)