Finding Similar Jobs
1 - keyword search. Take any word on the job description page that seems specifically related to the position and search for that specific word in job websites to see what comes up
2 - linkedin search. Take any word on the job description page that seems specifically related to the position and look it up on linkedin (may require pro account). This may lead you to people doing a similar job but with a different title. Which you can then do more research on.
3 - cold email search. Take the job you were excited about and try to find someone in that group or someone in the company and email them telling them you're interested in that job and would like to talk to someone at the company. Bonus points if you can find them on linkedin or twitter so you can see if they've written anything about it. Then ask them who else is in the field.
4 - [favorite search engine] search. I searched video game historian and found https://gamehistory.org/. Maybe they can be of assistance?
As for being marketable - first you have to figure out what the job entails before you can become interesting to people hiring for that specific job. If you do steps 1-4 above, you should have a set of tools, skills, projects that may be of interest to you that you can do to start a portfolio. Though realistically, it'd probably be better to just straight up ask them what you need to do to get a job there and then do that.