Someone I know has had the problem of trying to build muscle, below is the paragraph I told him on this issue. Please keep in mind I was taking to him as well as using his Stats.
Ok let me just clear this whole thing up... Listen closely because this is the truth and the only truth don't listen to other people that tell you to eat more **** because they are wrong.
Eating tonnes of **** is not good either way if your gaing fat or not. Its better that you were eating loads of **** and have fat on the outside as this is better. Eating tonnes of **** and not gaining "outside fat" is extremely bad as your body stores fat inside, this is the bad type of fat that leads to heart disease and blocked arteries etc.. Even though it doesn't seem like you're gaining fat your getting fat inside and becoming unhealthy. You are killing yourself and you dont even know it.
Eat healthy foods. Ok like say eat a chicken burger after the gym because you can afford this, eat two or three but make sure they have large chicken breasts inside. After the gym make sure to eat a massive meal macro rich in all 3 macros Protein fat and carbs.
You want to eat before the gym about an hour to 30 minutes before this will allow all the food you eat to be used as energy during the workout. Because you're 6ft2 and weight 50 ish KG you should eat 3500 calories per day you go gym doing this for one year will surly build you up to roughly 70k next year.
Don't ever eat **** food even though it "looks" like you aren't gaining weight. Don't even do this without going gym you are damaging your body more. Think about this, the harder you go at the gym the more you can eat? Go to the gym half ass is for an hour then eat 3000 calories, go to the gym and go all out rest 30 seconds become tired after every set eat 4000 calories per day. It's really quite simple.
Never fall into the trap of thinking you need to eat alot to gain weight. As soon as your body realises that your muscles are being used they will slowly use more "Fuel" to build them bigger. You really want arms that were build by MC Donalds? they will never be good.
Hope this helps.