Gunning joined BA more than 20 years ago prior to the formation of IAG in 2011, now BA’s parent.
He will be replaced as chief financial officer by Nicholas Cadbury, who will join IAG from Premier Inn parent Whitbread.
IAG chief executive Luis Gallego paid tribute to Gunning, highlighting his efforts to guide the group through the challenges posed by the Covid crisis, and said he still had a vital role to play in his remaining months with IAG.
Gunning will leave the business following the release of IAG’s full-year 2021 results.
"It has been a wonderful experience to be involved in the creation and success of IAG," he said. "I have also had the privilege of working with so many dedicated and talented people during my 20-plus plus years with British Airways and IAG.
"Having helped guide the group successfully through the challenges of the last two years, it is now the right time for me to seek a new challenge. The group is well positioned to take advantage of the recovery in air travel and to emerge a strategic winner in the industry."
Gallego added: "I would like to thank Steve for the key role he has played in leading the group through the unprecedented challenges we have faced in the last 18 months and wish him well for the future. His efforts and achievements have enabled us to navigate the pandemic better than most in our sector and ensure we are well placed to take advantage of the enormous opportunities as travel continues to recover in the coming months.
"In the short-term, our focus remains on delivering the best end possible to 2021 and ensuring we are set up to return to profitability in 2022, and Steve continues to play a key role in enabling the group to achieve this."
