To answer this question we need to first discuss how Garcinia Cambogia extract works for weight loss. Garcinia Cambogia has HCA in it which helps in suppressing appetite. This is why it is recommended to take Garcinia Cambogia extract atleast 30 min to 1 hr before your meals. When your craving for food is suppressed, you eventually will eat less. When one eats less body gets less calories and thus lose weight.
Coming back to the question. Suppose before you started taking Garcinia Cambogia extract you were taking 2,000 calories per day and burned just 1,200 per day. In that case 800 will eventually get stored as fat. With Garcinia Cambogia extract you will take lesser calories say 1,500, in this case you are taking just 300 extra calories. But you will still gain weight, though more slowly.
Why so? Remember, till your calorie intake is more than what you burn you will always keep gaining weight irrespective of whatever supplements you take, this is just simple logic. Taking extract will just slow down the increase. So in case there is a huge gap between what you burn and what you take exercising and a diet restraint can help a lot.
So all these claims by Garcinia Cambogia extract that lose weight without doing anything are wrong? Not really because fortunately, in most cases the gap is low and thus just taking supplements can bring down the calorie to a level where your body starts burning accumulated fats to satisfy energy requirements. But if you eat excessively then diet restraint will really help.
But if you can restrain your diet especially high fat food then you are more likely to lose weight and that too at a faster pace.