Grey hat SEO refers to tactics that are not necessarily unethical or strictly against the terms of service of search engines, but that may be considered to be on the border between ethical and unethical, or that may be in a grey area of what is acceptable.

Buying and selling links
Link schemes
Keyword stuffing

Here are some Gray Hat SEO techniques-:

Add Length to Your Old Content.
Buying Expired Domains.
Using Private Blogging Network (PBN)
Taking Advantage of the Right Keyword Density.
Careful Use of Social Bookmarking Sites.
Submitting Your Website to Web Directories
Link buying.
Social media automation and purchasing followers.