He has a habit of walking with his legs bowed outward.
People with bowlegs tend to put strain on their knees.
Common kanji-kana mixed spelling.
Katakana variant of the first part, often used for emphasis or in casual writing.
Ateji spelling using the kanji for 'crab'; less common but understandable.
The word がに股 is composed of がに (mimetic for a crab-like sideways walk, from 蟹 'crab') and 股 'crotch, thigh'. The exact derivation of がに is uncertain, but it is clearly associated with the image of a crab's legs.