Lost or misplaced your 2007 GMC Sierra 1500 owner's manual and need to find out your towing capacity? We have your owner's manual here. You can download it if you wish. Please note that this is a Dropbox link before you visit. The towing capacity information you are looking for resides on page 432.
If we could suggest a hitch for your trunk then we suggest that you use a Weigh-Safe trailer hitch if it works with your vehicle based on the information in the owner's manual. A Weigh-Safe hitch makes the job of getting accurate cargo weight measurements a breeze. All you have to do is load your cargo normally and then read the built-in scale to see if you are overweight or underweight. Please keep in mind that you should always factor in the weight of your passengers and yourself as well for maximum safety. The only WeighSafe models that do not have the scales are the 180 degree hitches, at least, at the time of this writing.
If you want to make your trips safer, easier, and less stressful then consider getting a WeighSafe hitch for your truck. Take a look at the products here after consulting your owner's manual about what hitch you need for your vehicle.