This map (1884/1887) shows that Gwern Berthi Road did not exist at that time except, perhaps, as a track. However a place named Gwern Berthi is marked (as Wern Berthi) in a position where the road would be in future years.

http://www.british-history.ac.uk/os-1-to-10560/monmouthshire/017

In this one (revised 1899) a road is marked but the terrace containing No. 29 is not there yet. It is not named but a place named Gwern Berthi is marked along its course.

http://maps.nls.uk/view/102186169

In the 1901 Census there is an entry (in the right place as far as I can tell) for,

Gwern Blethe Farm, Abertillery,
Simon COOK, 45, Mar, Farmer,

but no reference to Gwern Berthi Road.

In this map (revised 1915/1916) the road is named and the terrace containing No. 29 has been built. Reference to the farm has disappeared,

http://maps.nls.uk/view/102186166

I can track no reference to Gwern Berthi Road in the 1911 Census. Schedule No. 1 (Cottage Farm) jumps straight to addresses in Bryn-Morgan Terrace. My suspicion is that the terrace containing No. 29 was not built and the road not named until after 1911. This might mean a wait until the 1921 Census becomes available unless other documentary evidence is forthcoming.