As Elliot said some people are naturally lean (and skinny) and are lucky enough to have defined abs year round (sometimes regardless of what they eat).
Unless you are one of those guys, obtaining a six-pack and gaining weight naturally is a very very difficult thing to do. Instead, if I was you I would focus on one or the other. You can even do a "lean bulk" to gain weight and minimize fat gain by eating "clean" like you already are, just in excess of your daily maintance calories. Also, depending on how carb sensitive you are, you should also tinker with increasing your daily carb count. Do you have a high metabolism to begin with? You're on the right track Gunnar....my advice is pick one goal or the other and stick with it!