1. In Giliyang there aren't motorcycle/ something like that which pass on there.
2. There are a lots of lush trees which make the island have many oxygen.
3. On there, often occur a circulation of air near the beach 
4. The existence of caves in this island, according to 17 caves.
Grandparents are over 80 years old are still healthy fit. There's even a 175 years old. So, the travellers who visited there, will have a health body because the oxygen.

http://www.pulaumadura.com/2013/10/ekspedisi-wisata-oksigen-tertinggi-di.html

Gili Iyang gorgeous sandy beaches adorned with rocks. From the crevices of this rock, filled with morning sun. In the middle of a trip around Gili Iyang, you will see some whale bones that had been on display by the local people as a sign that their island ever visited by the biggest fish in the ocean.

https://seputarmadura.com/sumenep/berita-sumenep/menikmati-wisata-kesehatan-pulau-gili-iyang/

Other uniqueness of Giliyang are :
1. Giliyang has only one road around the island with a length of 10 km.
2. "Odong - odong" is the only vehicle in this island
3. The water for bathing not tasted so salty, even though near the sea
4. Power supplies will be turned off when daylight
5. There are 'Siwalan' (a type of palm tree), grow on this island and processed into sugar by the local people.

Sources :
http://javaisbeautiful.com/2014/10/17/gili-iyang-the-forever-young-island-sumenep-madura-east-java-indonesia/
http://www.wisatajatim.info/udara-segar-dan-sehat-di-gili-iyang-surga-baru-dari-madura/
http://www.aprijanti.com/2016/12/pulau-gili-iyang-madura.html