Yes: The Largest Ancient Stone Monument In Britain
OK, let's break it down.
Largest: Maiden Castle is the largest Iron Age fort in England, but Garn Goch's acreage is almost twice that of Maiden Castle's central area, and Caerau was said by Channel Four's Time Team to be the largest in Wales, yet according to Coflein (Wales' official authority on these matters) its area is 5 hectares whereas Garn Goch with its northern spur is almost 20 hectares. However, Garn Goch isn't an Iron Age fort, and there are at least two Iron Age forts with a larger area, but Garn Goch is 3,000 years older.
Equally, we are not claiming Garn Goch is Britain's greatest ancient stone monument, nor its most significant, nor its most important. That must be Stonehenge - no competition. It's much 'greater', but it isn't much larger.
Ancient: better than 'remote past', the most precise definition is before the emergence of writing, and therefore before written history. Literally, pre-historic, so the Bronze Age, Ancient Greece and Ancient Egypt are not, strictly, prehistoric / ancient. Neolithic, on the other hand, is indisputably prehistoric, so indisputably 'ancient', and the origins of Garn Goch's long cairn are indisputably prehistoric, so indisputably 'ancient', and the origins of Garn Goch's long cairn are indisputably Neolithic (constructed between 6,000 and 4,500 years ago), and is a westerly example of the chain of Neolithic long cairns that include Penywyrlod near Talgarth and Ty Isaf in Wales, and the numerous long cairns in the Cotswold-Severn group centred on Hazleton
Stone: there are a few larger ancient earthworks, such as 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.
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, for example, only qualify on the third of those definitions.
In fact, that definition excludes very large stone constructions like the 2nd century AD Hadrian's Wall, and even monuments like St Paul's cathedral and Wembley Stadium don't cover a greater area.
So, unless you've got evidence that there is a larger, more ancient stone monument in Britain, Garn Goch is (however surprisingly) it.