Short answer: yes, but its harder than gaining from synned foods.
Longer answer: I've only just joined slimming world but have been dieting for years and have done quite a bit of reading around the topic. If you eat excess calories, you can store them as fat, so ultimately, calories do matter. However, we very much oversimplify the calories in calories out model, because the are lots of things that affect whether calories are actually stored as fat, not just the amount of calories - hormones, nutrient profile etc. all play an important role e.g. Protein is generally used for growth and repair, and is less likely to be stored as fat unless eaten in large quantities. Sugar, however, causes insulin spikes and insulin is a hormone that promotes fat storage, so it is generally stored very readily.
In addition, our calorie calculations are very very rough - just because a food releases so many calories when burned in a lab doesn't mean it will do so in our body (researchers recently discovered, for example, that we can't access at least 20% of calories from almonds, so they're over valued). So there is quite a lot of leeway, if you are eating healthy food.
Finally, the body likes homeostasis (maintaining the status quo), so will reach set points with weight, where it will resist losing or gaining weight - for a while at least. so if you over consume at first, more calories will be "wasted" as heat or if you under consume more calories will be stored. We can change our set points but its why we tend to see weight loss in fits and starts and reach plateaus.
Basically, the healthier we eat, the more we can eat without gaining weight, which is why I prefer this to weight watchers where you can eat junk all day. but grossly overeating, even on healthy stuff will result in a gain.
Does that make sense? Sorry if I've oversimplified - anyone else feel free to correct!
