Re: gain muscle, keep fat off
If you have a "brilliant sixpack" don't worry about fat levels, especially if you want to gain some weight. Easiest thing to do is probably just have some more protein in your day and see how you go after a few weeks. Have another 100grams of carbs and 100 grams of protein a day, every day, and see if your weight goes up in 2 weeks.
"Builds fat" "builds muscle", you can get fat off eating a lot or you can get muscular from eating a lot, its down to activity levels for starters and a lot of other things.
But don't worry about that shit, yes its a very broad question.
Do as I suggest and see how you go, quality weight gain (i.e not bloat weight) takes a bit of time to come on. Keep your activity levels high, you can cut back on your running too if you like, I wouldn't interfere in stuff that would impede your performance though. Just add more quality food, and of course build some muscle with some strength training of some sort, but maybe you are one of these guys that eats **** all anyway and calls it "a lot" and "I can't gain weight", and your body is waiting for you to get some more food in to grow with your current training (65kg at 1,76 I'm sure you have room for growth).