What, exactly, do you want to do?
There's a lot of "gun related jobs" out there so narrowing it down a bit would help.
The easiest thing to get into is retail sales. If you want to be a really, really, good gun salesman learn more than *just* about guns. Learn about customer service and basic salesmanship. That's in additional to knowing enough about every general type of gun in the store to make recommendations. (Not every specific model, but you need to know something about defensive handguns, AR style rifles, bolt action hunting rifles, skeet and trap shotguns, BP, etc. Whatever the shop sells, you have to know SOMETHING about, even if you aren't "the shop expert" on that particular thing).
The reason getting into retail is the easiest is it's basically just getting hired into a sales job. Learn about guns, learn about retail sales, and start applying for jobs.
The bad news about retail sales is there is the hours, the pay, and the fact there isn't much upward mobility. But, if you think you might ever want to own your own gun store, it's a great training ground.
As far as gun writing goes, the best thing these days is to start your own website or Youtube channel and start doing reviews, how to's etc. There's some tough competition out there, but there's room for GOOD reviews, etc. The bad news is unless you become REALLY popular there isn't that much money in it.
As far as writing for the gun magazines, unless you are already a well known expert, or have substanial experience in some area, it's going to be near impossible to break in. The gun magazine industry is on a decline and there is less room for newbies than any time up to now.
As to being an instructor, learn more, get some NRA instructor certs, and go from there. Don't expect to make it a full time job though unless you are very lucky or unless there is some very strong demand in your area. (Like for state mandated CCW classes, etc). There are people who make a near full time living as local and regional area instructors, and plenty more who have a decent "side job" doing instruction.
Personally, if you are starting out, I'd say get a job at a local gun shop, learn a bit more about firearms and firearms sales, and also start your own website. You can look into becoming an instructor if that interests you as well. Start there and see what opportunities open up over time.
R.I.P HK33K - Gone, but not forgetten.