According to Horizon, the episode is 'Chip off the old block' which is on Wednesday 26 Sept, but the Radio Times has Gareth on the cast list on Friday 28 Sept which is 'Neighbours'. However, they do sometimes lump cast lists for the week together and every day of the week except Wednesday has a cast list, so Wednesday's cast list might have been added to Fridays. (It's supposed to be the 'regular' cast in the Friday listing, but I wouldn't be at all surprised if Gareth had been included there by accident)
If you want to watch it, I'd try Wednesday at 12.30 lunchtime and if Gareth isn't in that one, then try Friday at the same time.