(Edited based on added information from the OP)
As a DYI-er you could do gas fitting under supervision and inspection, and in readily accessible spaces. But if the space is tight, and you are unable to recognize from your own knowledge and experience what details for gas fitting must be provided up front in such a question, it's time to get a professional involved. As a DIY who hasn't done this you can't really see or assess what is fitted/sealed right/wrong especially in tight spaces, whether fitting are compatible, and whether any code or requirements have changed in your region.
- DIY but also get a professional involved (as you have now done - good move). Be prepared to pay for consulting and an inspection by the professional. It can be an informal "ok", but if you get an inspection letter that would be best.
The consulting up front will allow the pro to specify things like what fittings and materials you need and also to inform you about things you did not know that you had to know, like compatibility, pipe diameter, flow/pressure regulators, shut offs, and any upgrades that would make sense to implement now that you are involved with gas work anyway.
It is possible you have to do more work to replace other sections of the gas lines, and this is your chance to learn about it and do it right.
- The ugly part: determine whether you need a permit from your local authority and whether the home insurance requires an inspection (these two can be different requirements). This usually just requires a few phone calls. Then decide what you want to do with that information.
A pro would speak from tons of real life training and experience -not youtube-, and most importantly (s)he would bring liability insurance. Also, without a permit or at least a qualified inspection your home insurance likely will not cover this repair or install. This matters for peace of mind around your asset value, the risk you pose to loved ones and neighbours, and possibly the resale value.
Get educated (my personal first joy of DIY work). Knowing what work is required, what fittings etc..you are armed with the right questions. Read up on it, watch videos, ask at your supplier etc... when in doubt or whether information appears ambiguous or contradicting, check back with your pro. Since you have a consulting/inspection arrangement they are in the best position to give you advice they can stand by and sign-off on.
Prepare a test procedure and start testing: include pressure testing (if required), bubble testing, stress/torque testing etc... Since you have two shut off valves in the path to the fireplace, you can work in stages without shutting off the gas supply for the entire house.
Torquing should not be underestimated: some fitting require specific torquing to tighten for a gas seal, other fittings can and should sometimes be banged with a hammer on a wrench to loosen without cracking the pipe elsewhere. I'd check in advance.
Where the work is difficult to reach, first practice on the comfort of a well lit table or work bench. You need knowledge and skills when it comes to fittings, tightening and sealing, and in comfortable well lit conditions you'll have more patience to inspect thoroughly and keep trying until done right.
Have it inspected, and enjoy the Holidays in the warm glow of your upgraded fire place which you can proudly and confidently claim to have installed yourself, done right. That's the third joy of DIY.
Gas work, framing, electrical work and plumbing (esp. drains) all require meeting specifications and safety standards. The risk assessment is different for each of these areas. For instance, safety with electrical work can be met if rules and specifications are closely followed, and this site can help get you there. Also, problems with electricity often show up during use, in the presence of observant users who can take action to remediate. (This can be argued, but I hope my point is clear in light of DIY sites providing help with electrical vs gas work)
In contrast, gas leaks can happen regardless whether the fireplace is in use or not, and in spaces under floors and in walls. Moreover, failures occur like a time bomb waiting for windows and doors to be closed, enabling a fuel build-up. This type of hook-up is not the same as an outdoor barbecue or gas fire pit.
PS: oh ya, the second joy of DIY? Tooling up!