Göbekli Tepe has the monumental architecture of a true civilization, and must have taken many people, a lot of organization, and some specialization to complete, plus it appears to have been permanently inhabited for some periods.
The problem is, it's a neolithic site, presumably built by hunter gatherers. Hunter gatherers generally have sparse populations, move in small bands, with no permanent settlements, no leaders, no specialization no social classes and certainly no monumental architecture.
Yet there it stands, built by people with no metal, no pottery, no wheel, no livestock, no writing, no agriculture; who had to go out and hunt/gather their food every day. Yet they had the large numbers, organization, engineering and technical skills of a civilization, chiefdom or agricultural society.
Permanent settlements, large groups and specialization were thought to occur only after agriculture and animal husbandry created enough resource density to sustain a large, permanent population.