I am unsure if you are asking this seriously, or if this is supposed to be a joke…

but oh well, gooning is a term used to describe masturbation of an individual where they are so much into the practice where it becomes part of their daily life style, you could call it addiction, the term transformed partially into meaning just the act of masturbation. 

So Gooner bait, is an image, video, gif, text, etc. which purpose is to arouse someone and more specifically make them want to engage/interact with something, if it is a game they would want to play it, if it is a post they would want to comment or react to it in some way.

Specifically on this platform and on reddit it is usually a post of an attractive person/character which looks feminine (do note that it doesn’t have to be a cis woman) which is designed to bait the viewer to interact with the post, maybe a comment (maybe you have seen such a post with “reply XYZ and get a pic”) or a click unto OPs link which leads to a website where they could see the full (i.e. uncensored or fully pornographic something) sometimes through a paywall, sometimes to mine crypto coins, sometimes to show the person ads; and sometimes all of the above.

On lemmy and this community specifically it is usually some sort of homoeopathically sexual post so that OP can get some engagement for the “number go up” feeling.

If the comment was intended as a joke I do apologise for this lengthy comment, if it was sincere I should note that this explanation is based on purely my understanding

And I would like to refer you to https://www.urbandictionary.com/ which is a useful tool to try and understand some slang used.