Mr Grantham, who served 24 years in the Royal New Zealand Air Force, was shocked to be drawn from the ballot - as 9851 others had entered.
"It was quite a surprise," said Mr Grantham, who now lives in Taupo.
"So many people put their names in. It was like buying a Lotto ticket - you never expect to win.
"I was stunned. But, I'm getting used to the idea."
Mr Grantham's great-uncle George Prentice, born in Kent, England, in 1896, was one of the young soldiers who landed at Anzac Cove - and, unlike almost 3000 of his countrymen, survived the eight-month assault.
Six weeks after arriving in New Zealand in 1914, George enlisted as an infantry man in the Canterbury/Nelson Battalion and left for Egypt, in November that year.
It was there that he joined the Australian and New Zealand Army Corps, part of the allied forces Winston Churchill ordered to attack the Ottoman forces on land.
Hours after landing at Gallipoli, the Anzacs attempted to take a nearby hill, named Baby 700, but became caught in treacherous ravines and gullies. With the Turkish forces firing from above, "line after line" of Australians and New Zealanders were killed and wounded.
Mr Grantham never met his great-uncle but said he feels "disgusted" whenever he thinks of George's experience on the inhospitable Turkish terrain.
"He was lucky to survive," he said.
"The Anzacs were given a task that was beyond Superman. With that terrain, it was impossible.
"The politicians thought they were going into a flat paddock - they just had no idea.
"The whole thing was a needless slaughter."
Mr Grantham said he feels excited to visit Gallipoli.
"It will be quite emotional, I think," said Mr Grantham, who has copies of his great-uncle's medals which he will wear at the festivities.
"I would love to see where he fought - and have the opportunity to reflect on what went on."
