I don't understand why they have specifically labelled this as GWR. Did GWR cattle trucks appear on the show?
There's been three liveries on the Cattle wagons in the show:
There's the circus one, only seen in "Henry and the Elephant", as the ones from Season 8 onward use a different basis.
http://i.imgur.com/CdGSJEf.png
There's the standard brown one, with a dark roof. The one Bachmann originally produced. The last one in the train in the linked photo:
http://vignette1.wikia.nocookie.net/ttte/images/b/b4/Cows3.png/revision/latest?cb=20121020210859
And the last one actually appears to be in the colours of the new GWR one but in a much darker colour scheme, and with a different ownership name painted on the side rather than the GWR shirtbutton logo.
http://vignette4.wikia.nocookie.net/ttte/images/3/31/Cows07.png/revision/latest?cb=20160520034137
Maybe in future episodes, but must be to go with duck, Oliver and toad.