Can I use the current intake plenum gasket when replacing the head gasket?

No, you don't want to re-use gaskets because once they are installed, they get compressed, and they won't compress the second time. They do make some that are rubber that can sometimes be re-used but those are rare. However, you don't need two gasket sets, you could buy the head gasket set like the Fel-Pro HS9917PT3 set. As you are planning on doing the head gaskets you might consider going through the valves and seals on the heads as well. Oh, FYI that engine uses torque to yield head bolts so you will want to order those as well.
This is for an OHC engine but some of it is the same. https://www.2carpros.com/articles/complete-guide-to-removing-and-installing-automotive-engine-cylinder-heads

They are a bolt that is supposed to deform once they are properly torqued. They are not supposed to be re-used due to that stretching deformation. There are a number of videos on the 3.8 head gaskets and other repairs. The valve stem seals keep oil from getting past them. They are replaced by removing the valve springs and replacing them, then the springs are replaced and the upper seals that go under the keepers in the spring retainer.
For that you might want to look for videos that show rebuilding cylinder heads.
Like this one:

https://www.youtube.com/watch?v=fAKxSZ-qPIs

There are a lot of other informative items on this site as well as in the videos on here as well. If you have never worked on an engine before you will really want to look and learn.
Head gaskets are not a maintenance item so until they fail, or the engine shows excess wear they generally are not done. The other thing to consider is that most well-worn engines are also going to need some machine work to get them back into shape. Because of that you might want to price out a remanufactured or crate engine and compare that to the cost of the machine work and parts. That would get you a good engine and you could use the old one for practice. Or you could get a used engine for your car and go through it as a practice item.

I'm glad you realize that. Many don't and they will decide that installing a custom turbo kit over the weekend is a good idea, then discover that it's really a weeklong process that requires making parts to fit, or they will add something like nitrous and blow up the engine, then wonder how they can get to work. The big thing these days is that there are so many ways to learn compared to when I started. Depending on where you are there are also hands-on ways to learn as well, like visiting a you pull it yard and taking things apart there. For instance, you could go there and find a car like yours and remove the engine and rebuild that one. Then install it in your car. The yard near me sells engines for about $250.00. There might be a similar yard near you.

It's a great learning tool, especially when you don't have to be at work on Monday morning and discover you forgot to get some small part after 5 on Sunday! I may also suggest getting a subscription to either Alldata or mitchell1, they both offer single vehicle subscriptions that give you far more info and cover things that folks like me might forget to include because we assume it's already known.

I've seen manuals that way. The OE supplier for the GM manuals in that era was Helm Inc. Normally there were 3 or 4 volumes with one being the basic vehicle platform, another being in depth on assemblies and the others for unit repair (engines, transmissions). The thing is that most service information isn't written for a DIYer, it is written for a shop that does the work a lot and just points to the way the factory thinks things should come apart, with the tech reading it and filling in the blanks based on their training and experience. That is why you also don't see sections on how to use tools (other than specialty tools) and other "common" things. The tech is usually going to know these things (or should) before they get into the work. That is why there are different job skill levels as well. A-B-C being the common ones. A C level should be able to do any of the basic maintenance like oil changes, tire rotations and such. B level gets into lower level diagnostics and things like replacing window actuators, steering work, alignments and such. The A level folks can do all that as well as high level diagnostics, transmission, engine and more at teardown, inspection and rebuild levels as well as being the ones who usually mentor the lower level folks.
In the ASE world it would sort of be, No or only one ASE - C level, Most of the ASE modules and 2-3 years experience - B level. Full ASE master technician certification and 5 or more years on the line would be an A level tech. BUT don't take those as the gospel truth, I've seen and known folks without a single piece of paper who can work miracles on vehicles that made others cry, and I have seen people with every cert. they could get that I wouldn't trust to put the license plate on correctly!
Just a few folks who might teach you some tricks and methods
https://www.youtube.com/@SouthMainAuto
https://www.youtube.com/@Diagnosedan
https://www.youtube.com/@SchrodingersBox
https://www.youtube.com/@ScannerDanner

That is easy to do. Some of the sites I go to offer the same training I go to seminars for. I like the online because I'm not spending a lot of money (if any) but many of those classes also assume you are in the automotive trade. These are a couple of them.
https://www.youtube.com/@TSTseminars
https://www.youtube.com/@AutologicTV
https://www.youtube.com/@MotorAgeMagazine
https://www.youtube.com/@GoTechTraining

Most of the stuff is things I have had multiple classes on over the years. I just watch the newer items or refresh my memory on a topic. After 40 plus years working on vehicles and all the dealer an independent classes and seminar training classes most of it is in memory.