I actually done this without know it is a Gig. I was just running around the City while tranquilizing dudes out there. So it turned out that I "raised the alarm". I've done several side missions before I checked the Gigs again. Now, in my Journal it says to put the body in the drop point, so I tried to return to the location, and every body were nowhere to be found. And yep, I cannot complete the mission too.