Visited too long ago – 1999 – for posting relevant fieldnotes; however Coflein states:
A cairn on the W end of the main summit ridge of Gamriw, 20.4m by 16.6m and 2.0m high. A central crater houses a drystone building.
There are other cairns nearby – and of course the Carnau Cefn-y-Fford cairn cemetery lies in the valley to the south east (ish)