The chapel, completed in 1926, is the only National Trust building dedicated to the Great War which uniquely commemorates those who fell in the "Forgotten War" in Salonika, Macedonia.
It was built by John Louis and Mary Behrend primarily to house 19 large scale canvases by the artist Stanley Spencer, to honour those who died.
The new garden, designed by Hampshire landscape and garden designer Daniel Lobb and funded by a £100,000 Heritage Lottery Fund (HLF) grant, has been created to be a tranquil and special place for visitors to reflect on those who lost their lives.
The garden features fruit trees, scented cottage garden style planting and a vegetable plot, and was designed to complement the modernist proportions of the chapel. The original wildflower meadow at the front of the chapel remains.
Lobb said: "It was really important to me to quietly absorb the special atmosphere of the place and create a design that sits harmoniously next to the historic chapel, existing meadow and orchard.
"Taking inspiration from the strict formal aesthetic of the chapel building, the garden is rectilinear in plan and very simple.
"I hope it will provide both the opportunity for quiet reflection and an active gardening space for the various partner charity groups and volunteer gardeners which have helped bring my design to life."
Volunteers from the local community, horticultural students from nearby Sparsholt College , service men and women from Tedworth House, people from the London based homeless charity St Mungo’s and Thrive have put in 320 day’s work to complete the garden in time for the anniversary.