An easy GDP growth question A woman pays her gardener £60 per year and later married him. The gardener continues to work in the garden but unpaid. The woman earns £1000 before and after the marriage. Statement: the marriage decreases GDP by £60. 

My question is why the decrease? The economy still has £1000 regardless of whether it's consumed separately or combined (herself and her husband)?
You are confusing money supply with GDP. GDP is defined as the total monetary value of all goods and services produced within the economy. For this case,

GDP =  Woman's Earning + Gardeners Earning

Before Marriage = 1000 + 60 = $1060 


Even though the woman has $940 she was still compensated $1000 for whatever service she provided. The money supply is 1000 but GDP is not.

Post Marriage = 1000 + 0 = $1000