I know this does not match the condition on only speciality that were present in leat 10 editions, but I think 8 editions is relevant enough to be noted. Also, I am answering taking the sport at a whole, since checking 35 sports is easier than checking all its specialities.
China in table tennis seems to be the top one with 87.5% of gold medals over all specialities since its introduction in Seoul 1988!
In Table tennis at the Summer Olympics you can see that Chinese players have obtained 28 out of 32 gold medals and 53 out of a total of 100. The other gold medals were for South Korea (3, two of them in Seoul 1988, the other one in Athens 2004) and Sweden (1 in Barcelona 1992).
If you want to see some graphs on something related, you can check Sport Dominance: The percentage of events each nation has won per sport, in where you see archery (4/4 by South Korea), Table tennis (4/4 by China), Basketball (2/2 by USA) and Synchronized swimming (2/2 by Russia) are the sports in which a specific country has been dominant in Rio 2016.