Born in Chelmsford, Essex, and raised in Witham, Gibson's specialism is breaststroke and he competes in the 50 metre, 100 metre and 200 metre events. In 2003 he became world champion at 50 metres (long-course). He was 6th in the 100 metre event at the Athens Olympics in 2004. A year earlier, at the 2003 Summer Universiade, he won titles in both the 50 m and 100 m breast.
In choosing three words to describe himself, Gibson has said, "confidence, positive and pecs", referring to his prodigious pectoral muscles. On a 2003 edition of the sports-comedy quiz show They Think It's All Over he admitted to rippling his chest muscles before races in order to "freak out" the opposition.