**it happens...what you're describing is not abnormal for many cabinet smokers...my old GOSM 3405-GW was dinged up pretty good when I opened up the carton. My SV-24 had shipping damage as well.
If you don't receive a replacement door, I have some tips for straightening that out and getting it to close fairly tight. The weld-joint leaking up on top shouldn't cause much issue, as you need to vent the heat and smoke to keep the convection process of the smoke chamber happy, and to keep smoke from becoming stagnant and bitter. If it is a massive leak, say 1/4" wide and 1/4-1/3 the width of the cabinet, it could cause excess venting on that portion of the cabinet and result in some turbulence in the flow of heat/smoke...this may cause food to cook more quickly in that area, as well as have more smoke deposited on the food, while on the opposite side food may be exposed to less heat and smoke. The other possibility is getting precipitation into the smoker through that gap, but it would likely just drip down inside the cabinet, close to or directly on the cabinet wall, and not contact your food at all...I guess that could cause some grease to be washed out the bottom, prematurely.
When you set it up for smoking, be sure to level it really well and spread the legs out evenly so the cabinet is not twisted...this can make or break your door sealing reasonably well when closed. A few ounces of water in the pan will assist you in determining how level it is. Once you've got it level, you can determine if anything needs adjustment, like the door closer/latch, or maybe the door itself is a bit twisted. That's the main draw-back with cabinet smokers...they are very finicky about being level and having their weight evenly distributed on each leg.
Shout if you need help with tweeking that door...once I know what it looks like and how it fits into the cabinet when closed, I can help you straighten it back out and keep the damage to a minimum...fir or pine wood blocks (softer wood) can be a wonderful tools for situations like these. Rubber mallets make a great mate to the soft wood, too.
High-temp sealant should do fine for the gaps, but don't apply this until you have the door fitted as best you can...you may find that you must tweek the cabinet in order to get the door to fit properly...the sealant could prevent you from doing this.
Keep your chin up...things will start coming together soon, and you'll be enjoying the experience of smoking foods and dining on them!!!
Don't blame us if you get the mod bug and decide to transform your GOSM...
BTW, welcome to SMF!!!