How Old Is Garn Goch Then?
Let's focus on what is the oldest feature of Garn Goch: the main large burial cairn.
To firmly establish when it was built, we first have to know when it wasn't built. We've established elsewhere on this site that the Mesolithics built earthworks burial mounds on hilltops such as Trichrug above Garn Goch, in the Bronze Age they occasionally built comparatively small, always round, burial barrows topped with stone a metre or so high, and in the Iron Age they built forts to protect the living, not to commemorate the dead.
Therefore, large scale monument building - including large, stone long cairns - was associated with ancestor focused religion and funeral practices - and was an exclusively Neolithic 'aberration' dating to between 6,000 and 4,500 years ago, but we can do better than that.
We know that 'radiocarbon...suggests that agriculture first began to develop in this region...between [5,765 and 5,655 years ago with a] 95% probability', and that the teeth of some of the first Neolithic farmers, who were brought up in Brittany and buried in Talgarth, date similarly - 5,770 to 5,630 years ago.
So we can say with confidence that farmers were migrating to this region to open up new farmland about 5,700 years ago, give or take upto 70 years. The fact that those first farmers were buried in the Penywyrlod long cairn near Talgarth is compelling evidence that they immediately began building long cairns. Indeed, the authors of the journal article referred to above assume the contemporaneity of the cairn and the teeth, which would date the cairn to about 5,700 years ago.
Now two other points about the Penywyrlod long cairn: it is of similar size to Garn Goch's long cairn, and it is part of a group of Black Mountain long cairns that are associated with the dozens of long cairns centred on Hazleton that are part of the Cotswold-Severn group.  It is reasonable to propose, then, that Garn Goch is one of the most westerly of a widespread and numerous group of long cairns built in the early Neolithic, and that, therefore, it is not a random anomaly.
Consequently, we can also confidently date the Garn Goch long cairn to around 5,700 years ago, or perhaps 100 or so years later.
It might then have taken a few decades, even centuries, before the Cairn became sufficiently 'holy' to attract pilgrims from afar. After all, it was not until 1,000 years after the death of Jesus that pilgrimages to the 'Holy Land' became popular. If you accept that 'festival' attendees added one stone per ancestor every visit, then it could have taken hundreds of years to have assembled all those stones. One way or another, it seems highly likely that the 'ancestor stones' constituting the linear cairn were placed over centuries.
We also know that the last 500 years or so of the Neolithic era was dominated by a major decline of farming, and a rise in plague pandemics. People starving and/or being killed by plague do not build large monuments, so, making various assumptions, we can date the Cairn of Goch to 5,700 to 5,600 years ago, and the linear cairn assembly period to between 5,600 and 5,000 years ago, and perhaps a little longer.
Garn Goch, then, is older than Stonehenge.
But only archaeological excavations and technology based analyses will definitively prove whether that is roughly right - or wholly wrong - and we're confident Dyfed Archaeological Trust and Cadw will want to lead the way to that outcome.