I think when it comes to blooding them in the first team they very often do it at full back, as they see that as the "less risky" option, irrespective of whether they see their long term progression as full back or centre back. Even Unsworth did it with Feeney when he brought him on against Atlanta (one of Unsy's many weird selections in his short spell as manager).
I've only ever seen Gibson play centre back. I was away and missed the U17 euros and missed the 1 game he played at the subsequent worlds. But, to me, he looks an out and out CB whenever I've seen him for our U23s.
Having said that, given our current crisis at senior level I would give him a run out at left back. I realise there's zero chance of that under the current manager sadly.