While the original Japanese design for Gunvolt (as seen on left) had long hair, and his stomach exposed, the English version (seen on right) does not. His hair has been made shorter, he wears a shirt under his jacket, and some of the artwork's effects appear to be different as well. His electric sparks are made smaller and lighter, and his hair color is slightly lighter as well.

So, why was Gunvolt changed? Well, we don't know the answer to that, but one can only assume it's due to regional differences. Its possible that the developers felt that the Japanese Gunvolt with his longer hair would appeal to fans Japan, but not in the west (after all, he does look similar to other well known characters in Japan). Really though, it all comes down to personal opinions. Although, this wouldn't be the first time a character's design was changed to try and match the "trends" in different regions.
For more information, check out the official website, and check out the differences for yourself:
http://www.gunvolt.com/en/en_index.html