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