If They Are Not Fort Walls, Then What Are They?
If we argue that those lines of stones are not collapsed Iron Age defensive walls, then it is incumbent upon us to explain why they are there, and who put them there because there is no denying, and there is no reason to deny, that those stones have deliberately been placed around the perimeters of the site.
With a Neolithic mindset, the answer is simple and compelling because we know that for Neolithics timber was associated with life, and stone with death, but death has very positive associations for an ancestor worshipping religion. The ancestors were worshipped for their god-like powers to influence current and future events - from human fertility to good harvests - and they were venerated from self-interest - so as to stay on their good side, and ensure good things continued to happen.
How do we know this about a religion that has left behind few artefacts, and no written documents, nor even oral testimony? We know it because all religions are primarily about obtaining self-interested intercessions from gods or God that are believed to have the power, if they so will it, to change human destiny about matters small and large. Christians have God, the Romans had gods, and Neolithics had ancestors.
For Neolithics, laying ancestor stones around the perimeter of the sacred space was an act with double significance: it was at once a veneration of their ancestors, and a defining of the boundaries of the sacred space, each stone making it more sacred, more powerful. They weren't building fortifications to keep others out, they were delineating a powerful space for them to pray within.
So, the stones of Garn Goch belong to the ancient tradition of cairn building - stone upon stone, a personal act whereby each stone is individually selected and placed with no plan, no 'construction', just assembly. It is the religious landmarking of boundaries.
That's why the stones of Garn Goch are most accurately described as linear cairns or, perhaps even better, as one continuous linear cairn, albeit disrupted by entrances. Like other Neolithic monuments (stone circles, standing stones, henges), they represent a much more simple, more natural, and, arguably, more pure approach to religion and religious experience.
WHEN IS A 'POSTERN' NOT A POSTERN?
If it's a linear cairn, then how do we explain that single, anomalous 'postern'? The 'gateways' and 'double portals' don't need explaining because they are/were nothing more than wide open entrances, and even Hogg admits there is no evidence of them having defensive closures, but that one narrow, walled entrance/exit doesn't fit in with cairns and laid stones. It's clearly been planned and constructed, so when, by whom and why? There are, broadly, three possible chronological answers.
An Iron Age answer makes no sense because, although there is evidence of Iron Age activity on the site from the enlarged western walls, and they had a penchant for building posterns, why randomly build one postern on an otherwise abandoned site? Equally, the medieval (or anyway much later) farming answer makes no sense either because the 'walls' were not high enough to keep animals in, and an entry/exit like that was a huge amount of work if it's sole purpose was to let humans through, but not large animals like cattle and aurochs.
So, it makes most sense to go with a Neolithic answer, such as the 'postern' being a special, ceremonial feature where people would, for example, patiently wait in line to go through this scared space that had mystical significance. Let's hazard the guess that it represented the birth canal, and it led down to what was then still a large post-glacial lake where followed a baptismal type experience. Consequently, it had unique religious significance, so was a unique, one-off feature.
That it looked to Hogg like an Iron Age postern, although he admitted it had no signs of fixtures that would make it defensively secure, is fair enough as one narrow Neolithic entrance/exit built with stone will inevitably look like one narrow Iron Age entrance/exit built with stone. Not an answer that is wholly or immediately compelling, but at least it does not resort to using breeding rabbits as an explanation.