They were not. The primary stonework technique was almost certainly pecking [1], where you take a small "hammer" and whack it against the stone, pulverizing a bit at a time. This leaves a characteristic surface you can observe on the pillars today. However, the Gobekli Tepe builders were a bit smarter than just carving everything from scratch. They made use of the exposed limestone layers to quarry materials that were roughly the right shape and thickness.
People who shout about ancient aliens and hidden technology always completely ignore that the people in the past had access to the most advanced machinery available: humans.
Add time to that, so that your project can have a scope of generations, passing down knowledge gained from a lifetime of doing this particular thing, the results become unsurprising even.
Amazing, inspiring and ingenious but also unsurprising.
Abrasion carved both the high relief sculptures and cut the megaliths from which they arise?
Perhaps, I suppose. I don't know enough about the physics and control of the abrasive techniques referenced. Though, I'm always in favor of emphasizing plausible variables that weaken a hypothesis.
What are the chances that abrasion work like that, hypothetically, was so specialized and unknown in 10,000 BC that Gobekli Tepe's high relief sculptures are the sole examples of that period and the earliest examples of such large and truly technical work? Nevermind the artistic skill that is reflected.
Cro Magnon was carving anthropomorphic stone statuettes in the region of modern Germany in 40,000 BC.
Food for thought: the Tell archaeological sites that dot the Fertile Crescent, which includes the Haran area (Gobekli Tepe), contain the most concentrated evidence of human technological firsts that seems to exist. Including evidence of earliest metalsmithing tech, if I remember correctly. See Yarim Tepe for a jumping off point.
So the situation is:
a. Stone carving predated Gobekli Tepe by at least 30,000 years
b. The earliest known stone carvings were not in the region of Gobekli Tepe. Implying that stone carving culture and tech was known far outside of Mesopotamia, from an extremely early period.
c. Gobekli Tepe's specific accomplishments are the first evidence that we have of such advanced sculpture and megalith construction skill.
d. Metalsmithing arose in the immediate regions and cultures that included the area of Gobekli Tepe.
So on one hand we have widespread tech suggested to be responsible for the first evidence of highly technical stonework that is limited to one area in this early period of pre-history, and on the other we have specific advanced tech (metallurgy) that is best for such technical and large scale work arising out of that same area.
It could be a coincidence, but arguably it would be a large one.
Metal is too precious to be just abandoned. Metal is usually recycled, even in our civilization.
Whatever metal objects from ancient civilizations we know, are either grave goods (swords of warriors etc.), or were found in shipwrecks etc., or were found in destroyed settlements, plus a few accidentally lost (fell into a well etc.). But most metal objects get recycled.
[1] https://stonetoolsmuseum.com/technique/pecking/