Whether he's saying "g'down" or "go down", it's simply not a long enough snippet for me to accurately make out an accent to determine from where he may come. To get a full idea of an accent, you must have several sentences in which to weigh speech pattern, cadence, dialect, and pronunciation. The sentences would preferably vary in type: a question, a command, an exclamation, etc. I have two degrees in Anthropology, including a specialized BA in Linguistics (my other is in Religious Anthropology), and I wouldn't jump to make any conclusions based on such a small snippet. Without context, it's hard to tell if the shortening and/or "blurring" of "good" in that sentence is an indication of an accent, speech impediment, or if he stumbled while taking a step and it just came out wrong.
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