Here’s a quote about Cerrig y Gof from the book Prehistoric Preseli, by N.P. Figgis:
There are a few other tombs of vaguely similar pattern in western Scotland and eastern Ireland, but they are rare and none is really very similar. Fenton, the early nineteenth-century Pembrokeshire antiquarian, explored [Cerrig y Gof] when it was already in much the same state as it is today, and found some black pebbles (unusual – white quartz is common), charcoal and bits of unburned human bone along with what he described as sherds of the ‘rudest’ pottery, all now sadly lost.
Here’s more from Neolithic Sites of Cardiganshire Carmarthenshire & Pembrokeshire by G. Children and G. Nash:
Cerrig y Gof was excavated in 1811 by Fenton... he believed a central cromlech originally completed the complex. This argument is disputed because the central area is too small to accommodate a chamber of a size similar to the other five.
These two books are excellent resources if you’re planning a trip to the area.