Here is the full list of all 20 Guard Duty achievements.
End it, once and for all
Rendezvous with a familiar face
Something amiss
Never give up
Insult to injury
Find a way to open the door
Meet the Guardians
Venture North, beyond the Scar
Return the paint to it's rightful owner
Deal with the Spider
Fail to make it across the Elder Scar
Get the Wildlands DLC
Successfully snap a photo
Translate the creepy book
A book bound with flesh
Deliver the news to King Theodore
Find a quiet place to get dressed
Girtrude finds a new friend
Make a charitable donation
Get kicked out of the Castle