Brands dont much care how they get heated, so long as all of the branding surface gets heated. Pay close attention to the first line of Mr Bengals reply, "you'll want all of the brand to be hot enough at the same time". That will limit what heating method you use depending on how large the brand is.
Smaller brands, say the size of an index card (roughly 4x3") would be the limit of what i would say you could do with an average blowtorch. A gas grill probably wont work too well, even at the index card size. The burners in those are meant to output a lot of heat, sure, but theyre meant to do so over an equally large area. It would be a challenge to get a brand up to a working temperature, not worth it in my opinion. A charcoal fire would be the best option, for nearly any size of brand too, so long as you have a big enough spot to build the fire. Get a fire big enough to fit the brand going, stick said brand down in the coals, get it good and toasty and get to poking.
Oh, and whatever you do do not use an actual project as the victim for your first attempt at branding. Brands on wood can be a bit finicky, it takes some experimentation to figure out how hot you need the brand and how long you need to keep it on the wood. Too hot or too long and you end up with charcoal. Also, branding goes before finishing, DAMHIK