Grey hat SEO techniques are a combination of Black Hat SEO techniques and white hat SEO techniques. It is basically making use of white hat techniques up to a level where it is just about to become a black hat SEO technique.

Grey Hat SEO is the practice of using technically legal methods to improve your site rankings, but which are ethically dubious, and could one day become black hat.

Combination of White Hat and Black Hat SEO is Gray Hat SEO,
Remember one thing Google or other search engines are only gives priority to White Hat SEO.

Gray Hat SEO Technique combination of both techniques White hat seo and black hat seo techniques. SEO which stands in the middle of White Hat SEO and Black Hat SEO.

what is site mapping?
 

what is site mapping?

It is mixed from blackhat and white hat

Grey Hat SEO is the practice of using technically legal methods to improve your site rankings, but which are ethically dubious, and could one day become black hat.

It's black hat seo that's not forbidden by google YET.

Gray Hat SEO is an SEO process that's more risky than White Hat SEO, but one that may or may not outcome in your website getting suspended from search engines and their affiliate sites.