There are apparently two monuments here, maybe both "hengiform", maybe one is a cairn, with maybe a standing stone near to the northern one.
All I can say is there's a lot of trees and vegetation in these parts. And a stone.
GGAT and Coflein have the following, somewhat contradictory records:
Graig y Gilfach 1 (north) (SO02940346)
This cairn was described by RCAHMW in 1960 as 'a cairn 6.4m in diameter with a flat, turf-covered top, built of loose stone on a natural boss of rock. Beside it on the W is a large block of stone (? an 'outlier') on which most of the outline of a millstone, 1.2m in diameter, has been shaped to a depth of 0.10m'.
Poor GPS coverage on day of site visit, but NGR appeared to be in quite a large area of fallen trees. This was searched as well as possible, but no cairn could be seen - possibly concealed below fallen trees.
The site is likely to be a hengiform monument (Wiggins and Evans 2005)
Remains of a round burial cairn situated within forestry on the edge of the summit of Twyn Gwersyllfa. The grass covered cairn is circular on plan and measures about 9.5m in diameter and up to 0.7m in height. The cairn was originally scheduled in 1961; however, the area covered by the original designation did not relate accurately to the remains on the ground.
Graig y Gilfach 2 (south) (SO0300103375) GGAT
Roughly circular monument consisting of an external bank, now spread and indistinct and clearly visible only at the W and N, with an internal ditch 0.4m wide and 0.3m deep. Most of interior falls within a clearing, which also contains the W side of the bank and ditch; elsewhere the bank and ditch are obscured by overhanging trees or have disappeared; young trees are self-sown in the SE quadrant of the enclosure. Classified by RCAHMW, with reservations, as a ring ditch, but actually appears closer in form to a henge, though because of damage to earthworks and low overhanging branches it was not possible to be certain whether there was any entrances. RCAHM however noted in 1960 that the bank was in ten straight sections. Dimensions: 25m diam; ?c0.3m high.
A subcircular enclosure, 21.3-24.4m in diameter, defined by a ditch with a slight external bank.