There is no better way to learn GCode (and Mach 3) than by sitting down and doing it.
Basic GCode is very simple. The X and Y axis have co-ordinates, and a line of code tells your machine to move from where it is (it knows where it is) to a new location. That is it.
If you mention one axis in the line, than one axos moves, if your mention two axis, then both move simultaneously in time with each other, and if you mention 3 axis at one time you are better than me - but the same rule applies - all axis move simultaneously.
The move can be at hgh speed - i.e. what your motors are set at for top speed by using G0, or you can use G1 where you can specify the speed the axis(es) travel at - using F (Feed) in units per minute.
The codes G2 and G3 are for cutting an arc. This is a circular arc, as opposed to a hyperbola or other type. Here, as before the finish points are required, together with a radius (R parameter) or a centre paramter (I or J or K).
These codes are all that is required to make your machine do meaningful work. The other codes listed are all to do with offsets, or specialist drilling moves, or metric and imperial, etc, to make the first four codes do the job at the correct place - and, quite frankly are ignored by a lot of the Cam programs, which simply write what they want to do in the first 2 codes, G0 and G1.
Get stuck in and sit down and write some GCode program for yourself. Get some graph paper, which gives you a co-ordinate system, and draw out on that, where you want your tool to go, and then write the GCode to match it. You will be amazed that you can achieve so much with so little.
As you progress, you will investigate if there is a more efficient way of writing the Code, and there might be, but as I said before, many of the CAM programs are not efficient, they stick to writing things in lots of little lines.
The most important thing about all this is to make sure that you write your program, from a point 0.0.0. This is a point that you can set on your machine and know that when you start your program, the machine knows where it is.
One a lathe, X0 is normally the centreline of the lathe bed at rotation height, and Z0 is normally the right hand edge of the workpeice, both things you can easily find.
On a mill, X0Y0 is normally the bottom left hand corner of the workpeice, with Z0 being with the tool resting on the top of the workpiece, again a place that you can easily find - and more important in both cases - you can set your machine to.
These places are known as Work Co-ordinates. The machine itself always runs in Machine Co-ordinates - and the Mach DRO's display either. As you get more proficient, you will learn (or should I say - come to understand) how you can have the machine set itself to home co-ordinates, then apply offsets to bring it to the correct place to begin work. That, however, is a fair way in the future, and something you do not need to know at the present time.
Keep it simple at first, have a go writing your GCode and you will learn very quickly.