Smoke leaking out the door is not a major problem but I smoke a lot in cold weather and every little bit to keep the heat in helps. Ideally, you want the smoke to go up, around the meat, and out the stack/vent.
On my GOSM, I used the Red high-temp RTV silicone gasket maker (any auto parts store & Kmrt, Wmart have it low cost ~$4.00 a tube). It is rated to 650 degrees and specifically says for oven doors on the package. You need to clean the edges off thoroughly before applying. I recommend applying it to the box frame, not the door. I used some other silicone junk my first attempt and only used masking tape on the door to keep the silicone from sticking - long story short, the door didn't want to open and I messed it up and started over. Second time I still put masking tape on but then carefully placed plastic wrap over masking tape and it worked much better - door opened easily after it cured. I used 2.5 tubes on my door (mine is medium sized GOSM) and I expect you will need 4 of the 3 oz. tubes for a big block. I generously applied the gasket maker to the box frame and carefully closed the door and latched. I waited 2 days before opening and then used a utility knife to trim away the excess.
It is still not absolutely leak proof but way better than without. You should be careful with GOSM doors on windy days. It got away from me one day and swung around hard and bent the hinges a bit and the door sagged enough I had to lift it to close it. I had to take the door apart and bend the hinges back. If that happens after you apply the gasket, it may cause it to not seal as well.
Here are pics of my gasket: