These claims sound specious. And I don't know what bitter gourd tastes like, but it doesn't sound too good.
Eat a balanced diet high in fiber, focusing on fruits and vegetables (especially leafy green vegetables), low in fat and cholesterol, and where the carbohydrates you take in are not simple sugars or starches, but rather complex carbs (like beans) and / or whole grains.
Certain types of fruits and vegetables have additional beneficial properties. Grapes are high in flavinoids, cranberries are good for bladder health, and leafy green vegetables can help provide your body with nutrients critical to brain health, specifically functions like memory, etc. But if you're eating lots of fruits and vegetables anyway, the chances are good you'll hit quite a few of these -- and it's better than focusing on one food to the exclusion of another.
Good luck!