Here is the full list of all 11 Gunnheim achievements.
Find all weapon drops
Find Totem in Masons Grief
Find Totem in Turkuld
Find Totem in Forest Of Hayr
Kill Odin in normal mode
Kill Odin in hardcore mode
Complete Forest Of Hayr in hardcore mode
Complete Masons Grief in hardcore mode
Complete Turkuld in hardcore mode
Complete Muspelheim in hardcore mode
Find Totem in Muspelheim