Googling it led me to a Mail Online article explaining that gastrosexuals are in fact men who:
consider cooking more a hobby than a household chore and use their kitchen prowess to impress friends and prospective partners
The article indicates that the term comes from a study commissioned by food company PurAsia and entitled 'Emergence of the Gastrosexual'. This is downloadable from PurAsia's website, which is – ridiculously – under construction (see below). The site's URL (www.gastrosexual.com) indicates that the company is making a big deal of its neologism.
I'm not sure what PurAsia is trying to sell – Asian food aimed at men in some way? But whatever it is, it is "currently exclusively available at Tesco Extra, Tesco Superstores and tesco.com" so maybe I should go and check. Or perhaps PurAsia should have got its website up and running before releasing the report and attracting interest from the mainstream media (and me).
So what does this all prove? That if you want people to read your study, coin a word – even a lame one. I mean, it doesn't even rhyme with 'metrosexual' or any other '-sexual' word.
Another thought: PurAsia is an unfortunate name for a food company. It reminds me of Purina.