Re: any gymrats?
Just general stuff as I work at a gym (accounting) and workout about 4 times a week, but still not a gymrat and not an expert or anything, but I can relate to your story.
I graduated college in 2009 still weighing 155lbs and I am also 6'3. After my dad passed away that fall I got really serious into health and fitness for a while as a release. I focused on compound exersizes to build mass (Squats, Bench press, deadlifts, cleans, pullups, etc)
People of our build really have to try hard to put on mass and that means you are going to have to really eat. I know you probably think you eat a lot as I did too, but I put myself on a 4,500 calorie a day diet and made sure to lift heavy 3-4 times a week doing low reps and going for volume (5-6 sets).
You need to keep track of every single thing you eat and drink and log it (there are plenty of apps out there that can help you with this on a smartphone) to make sure you are hitting your food goals that day and also entering in your lifts so it can keep track of how many calories you are buurning too. Just a note it's easy to get to 4,500 calories eating junk, but i'm sure that's not how you want to do it. You are going to be eating when you aren't hungry on healthy foods just to meet your goals
(cans of tuna, grilled chicken breast, eggs, nuts, all kinds of fruit, broccili, etc)
Be sure to get plenty of sleep too. Eat a big breakfast and afterwards drink a shake, bring some healthy snaks to work and eat them throughout the day while drinking tons of water, healthy lunch of chicken breast, veggies, and then protein shake before and after workout, healthy dinner, and then a casein protein shake before bed
Those are just some things I did I didn't always eat healthy as I was ok with adding a bit of fat to get to my goal and then change my diet and cut some once I got to my goal weight. I now weigh 190 but I did this in 2009 and got up to 190 by the end of 2010. I now just kinda maintain and don't train as hard or eat as healthy as I did then