Author Roderick Floud has something to say about how GDP is calculated:
Activities such as housework, looking after children, or decorating homes, together with the gardening or the value of the fruit, flowers, and vegetables that are produced from it, are not counted. When there were plenty of paid gardeners, their wages would be counted as part of GDP; today, when it’s largely done by the householder, this time and effort are not.
So the role of gardening in the economy is much understated because it is largely excluded from GDP. Perhaps equally serious is the fact that few people realize how much land is taken up by gardens, nor the value of that land if it could be used for other purposes.England’s Magnificent Gardens, p. 345