John will be joined by friends Mark Taylor and Alan Granger for the 27-mile challenge, which will include 72 holes at the Carrick Golf Club at Cameron House Hotel on the banks of Loch Lomond.
The event, on June 19, will see around 2,000 teams across the UK take part in the Longest Day Golf Challenge to raise money for the cancer charity.
John said: “There aren’t many people whose lives haven’t been touched by cancer, either directly or with a loved one or friend. It’s estimated that one in three of us will be affected by cancer at some time in our lives.
“We’ve seen first-hand the amazing work that Macmillan nurses carry out and so once we heard about the event we wanted to take part.”
Professional players from Carrick Golf Club will join the team of three throughout the day and John has been preparing by fitting in an impressive three rounds of golf a week.
“I play every Saturday and Sunday, plus at least one evening now we have the lighter nights, but I still expect the event to be a real endurance test,” John said.
“I’m hoping we can raise £500 for the charity, and we’re already more than 70 per cent there.”
In 2015, the Longest Day Golf Challenge saw 2,000 teams take part, raising an impressive £1 million for Macmillan.
The cancer charity provides specialist health care, information and financial support to people affected by cancer. It receives 98 per cent of its money through voluntary sources, with a total of £47.8m raised in 2014 through fundraising events, such as the Longest Day Golf Challenge.
To support John Gunn in the Longest Day Golf Challenge, go to: www.justgiving.com/fundraising/John-Gunn5