I used to live in Maesteg which is in the Parish of Llangynywd. There is top Llan, which is the village with the Church and some medieval buildings. and obviously some later houses. Then there is bottom Llan, which possibly could have been known as Lower Llangynwyd. As you drive from Maesteg towards Bridgend you come to Cwmfelin and on your right hand side there is a road going up a hill to top Llan. The road goes straight up and when you get to the village the road goes straight through with the Church and older properties including two pubs, being off to your right. After the road passes the village it then goes down a hill until you get back on the the road to Bridgend that you were originally on. This is bottom Llan. The houses, if my memory serves me right, are mainly 1930s? in what was originally social housing. Though there are some possibly Victorian houses at the bottom of the hill.
The main road follows on for possibly at least a mile or even two miles before you get to Coytrahen. So I would think that it is probably bottom Llan you are looking for. Also when you come down the hill to the main road instead of turning right towards Coytrahen and Bridgend if you go straight over you go down another hill with some older houses down to the Llangynwyd railway station.
Just had another thought if it is Coytrehen, there is or was a very nice cottage on the main road nearer to Coytrahen but I can't remember if it is opposite the Paper Mills, which is on the Llangynwyd side before you get to Coytrahen or if it is nearer the Coytrahen side before you get the Paper Mills. Off course it could have been demolished because I have not been back there for years.Again this is on the right hand side as you go towards Bridgend. I also have a vague recollection of reading that the cottage was to do with Cefn Ydfa Farm which is near to where the cottage is, so could possibly be your gatehouse. Hope this helps.