Unfortunately, the 1831 only listed one name from household and then total numbers of male/female/servants, so at best is only an indication of possibilities. For example, the other male could have been an adult sibling of Paul, an unrelated boarder etc.
Purely based on the not overly common surname in the timescale/area and there being others in the same parish, it is possible there were 'cousin' relationships around. I think it is more likely this is where Hugh would fit in. That is to say, possibly as a cousin to Martha, albeit there may have been no connection at all.
I would definitely try to pursue the other Gurney, ie William, who was in the same townland as a Paul in 1831, as judging by property numbers, they seemed to live near each other. Overall numbers of any surname in the townland were small, so I would think there was a high chance of a family connection between the two Gurneys.
Yes, it can at times be frustrating and/or confusing and often it is unfortunately impossible, in part due to lack of records in some areas, to ever make complete sense of a particular family group or their extended family, but it is always worthwhile extending searches out to 'nearby possibles', as sometimes they can ultimately provide a clue or missing link or if nothing else, perhaps even be eliminated entirely.