Actually there is no said definition for Gray hat SEO, but in practice it is a Search engine optimization method which uses both the practices from White Hat SEO and Black Hat SEO methodologies. So we may say that Gray Hat seo comes in between Black Hat and White Hat Practice.
Gray Hat seo although followed today by most of the digital marketing agencies, it is a search engine optimization practice which should be approached with extreme care. Although it doesn't uses all the practices followed in Black Hat seo, a practice which can get you banned in search results , it still is a risky approach to optimize your website/blog/eCommerce store with Grey hat search engine optimization.