Gyōkai is a Chinese-character compound (on reading) overwhelmingly written in Chinese characters as 業界. The component meanings of the characters are 'industry, profession' + 'world, sphere', in other words, 'the sphere of an industry' or in everyday parlance, simply 'the industry' or 'the trade'.
One would be hard pressed to think of a reason why anything but the characters would be used to write gyōkai. But the example below shows how flexible the Japanese writing system can be: gyōkai can be found written in katakana as ギョーカイ:
This example comes from one of the ubiquitous advertisements for the coming issue of popular magazines that hang from the ceiling of commuter trains. The headline encourages hopefuls to plunge into the calling of their dreams.
Notice how the verb akogare 'longing' is written in Chinese characters as 憧れ, as is the word shigoto as 仕事 -- but not gyōkai! It's not clear why the writer has chosen to single out gyōkai for using katakana. Possibly it's an attempt to distance the article from the image of the word gyōkai, with its associations of stiffness, ingrained practices, and lack of imagination. It seems that even the word for 'industry' or 'trade' can be made sexier by writing it in katakana!
A Google search in August 2003 revealed the following distribution of forms on the Internet:
The Chinese characters 業界 are clearly in no danger of being replaced, but it is interesting that there is even a minority interest in writing gyōkai as ギョウカイ or ギョーカイ.