The Largest Ancient Stone Monument In Britain?
OK, let's break it down.
Monument: here's Oxford Dictionaries' three definitions: a) a statue, building, or other structure erected to commemorate a notable person or event; b) a statue or other structure placed over a grave in memory of the dead; c) a building, structure, or site that is of historical importance or interest. On all three definitions, then, Garn Goch, or at least the long cairn, is a bona fide monument, whereas Iron Age forts only qualify on the third of those definitions.
Stone: there are lots of larger ancient earthworks, such as Avebury and cursuses running for miles, but we're only talking about stone monuments. In Brittany, Carnac's lines of erect stones run for kilometres and kilometres, but we're only talking about Britain.
Largest: Maiden Castle is the largest Iron Age fort in England, and Caerau is said by Channel Four's Time Team to be the largest in Wales. The area encompassed by Garn Goch's stones is larger than both, and anyway neither are made of stone. Garn Goch's acreage is almost twice that of Maiden Castle's central area, and Caerau, according to Coflein (Wales' official authority on these matters), is 5 hectares whereas Garn Goch with its northern spur is almost four times larger at getting on for 20 hectares. Unfortunately for comparison purposes, though, Garn Goch isn't an Iron Age fort.
Greatest: of course, Garn Goch is not Britain's greatest ancient stone monument, nor it's most significant, nor it's most important. That must be Stonehenge - no competition. It's much 'greater', but it isn't much larger.
Ancient: the origins of Garn Goch's massive long cairn are Neolithic (say 6,000 to 5,000 years ago). It's a westerly example of Neolithic long cairns including Penywyrlod near Talgarth and the numerous long cairns in the Cotswold-Severn group centred on Hazleton. That means it's about 2,000 years older than the Iron Age.
So, unless you've got evidence that there is a larger, more ancient stone monument in Britain, Garn Goch is (however surprisingly) it.