Excess calories put weight on,nothing else.
Fat can be your friend,or enemy,depending on what type and when it's incorporated into your diet.As an energy source when carb depleting it's very useful and satisfying.With regards to fructose,it may be lower on the GI scale than glucose or sucrose but its still a sugar that,if eaten in any great quantity,will quickly and easily(especially when liver glycogen levels are full)will be converted and stored as excess.As such ive always found fruit a good addition to a diet but certainly not something i'd use as a staple,where as green veg reacts very differently in the system and can be eaten in much greater quantities.
The GI/GL diet,coupled with eating little and often,is the ideal approach and weight maintenance plan IME and opinion.