Generally, Gray hat is the combination of Blackhat and White hat SEO. Gray refers that method are not fully disapproved to use by Search Engine, but still not considered to the White Hat SEO. But using gray hat techniques improve your site ranking without negative consequences.
Gray hat SEO is a set of techniques and strategies that don't generally adhere to Google's guidelines but can still be considered acceptable. It is the middle ground between White Hat and Black Hat SEO. It is technically legal, but risky
Gray Hat SEO is an SEO practice that's riskier than White Hat SEO, but one that may or may not result in your site being banned from search engines and their affiliate sites. Generally, Gray Hat SEO practices belong in neither the Black Hat nor White Hat category because the terms of service regarding the issue are unclear.
Grey hat marketing is a combination of commonly accepted SEO techniques, such as paying someone to write an article for your website, and banned practices known as "black hat marketing," such as hiding keywords in a page by making them the same color as the background.