I agree that the rails could well be from the original Nantlle railway.
This was converted from 3' 6" gauge to standard gauge in 1868, presumably using more modern rail, and so it is likely that a considerable amount of second-hand wrought-iron rail would have been available at that time.
Richards in Slate Quarrying at Corris (p. 45) suggests that the rails at Gaewern came from the Upper Corris tramway. However, Boyd in Narrow Gauge Railways in Mid-Wales (p. 39) states that the branch was laid in bridge rail.
Of interest to me is that there is a length of similar rail at Cilcwychwyn manganese mine. This mine was working in the early 1890s. It had already been suggested that this rail might have come from (in order of decreasing probability) the Ffestiniog or Nantlle railways, Gaewern Quarry in Corris or Glynrhonwy. It's starting to look as if it could have found use at several of these enterprises before finding a home (in a very worn state) as a gatepost in Cwm Nantcol.
Anyway, thanks to Tamarmole to drawing my atention to Losh (who I'd not heard of before) and Stephenson and which led me on to Birkinshaw and his wrought-iron rail.
Dave