The G4003G is a good machine to start with, I have one and it does nice work when good tools are used.
It comes with a QC Tool Post, buy 2-3 extra tool holders for it to start.
Throw the basic parting HSS parting tool into the recycle bin and spend the money to get something that holds indexable carbide. Blade or bar style is pretty much up to you, I have one that's made by Iscar and uses double-ended carbide inserts.
Carbide insert tooling is expensive compared to cheap HSS cutters but they're worth every penny IMO. When I buy a tool holder I also make sure I buy a 10pack of the cutter to go in it for what I'm doing. It basically doubles your initial cost of the tool but you now don't have to worry about the "awwww F*** me, I just chipped the last corner on the last insert" until you've had more than ample notice you're running low on inserts.
For basic gunsmithing work there's only a couple places that I don't buy [asian] import tooling and that is the laydown threading inserts and insert holders, and carbide boring bars for doing tiny work like muzzle brakes.
I try to use carbide or indexable carbide whenever possible for milling, turning, and threading.
All of this can be done through judicious use of Fleabay, like today I picked up 5 brand new Micro 100 BB-2001250G carbide boring bars for $62.50 shipped. Compared to MSCDirect that's about 70% off.
Also, get a copy of
Machinery's Handbook and practice calculating feeds and speeds for steel, stainless, brass, and aluminum. These are things that will help you immensely when getting started on chip making. Plastic rod like Delrin or unreinforced Nylon (the glass filled stuff will kill your tools) are both good for practicing on the machine as you crank the knobs around and cut threads and things like that.
If you're not thinking "ok, now, which way do I throw that lever or turn that dial" you're much less likely to smash something up front where serious damage is caused to expensive tooling, machinery, or parts.
Finally, Youtube is a great resource for learning basic and even some advanced techniques and tricks to making better parts and cutting time.
Good luck with it.
ETA: A good set of calipers, a pair of decent micrometers (0-1 and 1-2) as well as a jeweler's loop, thread setting gauge, protractor are all something that should be considered basic tooling that cannot be ignored.
I use a long reach indicator stem on a test indicator and reach down the bore for doing barrel work, I don't like the marginal repeatability of range rods.
