it is actually true that a big grapefruit would have more total sugar than a mars bar, but a mars bar has more added sugar and that's what the system should be concerned about. i'm actually trying to rationalize my argument, as you eat a chocolate bar all at once. you don't drink a 3 L container of juice at once. 

the glycemic index is more important:

mars bar - glycemic index of 62, glycemic load of 20

grapefruit  - glycemic index of 25, glycemic load of 4

3 L jug of orange juice - glycemic load of 125, but you don't drink it at once

500 ml glass of orange jucie - glycemic index of 40, glycemic load around 15

glycemic index of 55 or lower is low.

you should eat the grapefruit, unless you just had a blood test and fasted the last 15 hours.