Bulent, when I learned Gmax I began with the many tutorials, just like you have. What I was doing was pure imitation, felt slightly brainless, and stopped the tutorials frequently while I figured out what I had done differently than the teacher. After a few weeks, something happened. Perhaps "epiphany"? Anyway, I started to really get the structure of the program...what the stack really represented.
Gmax was built from version 4.0 of 3dsmax. While long in the tooth, version 4 of 3dsmax was quite advanced from the beginning of max's life. Max has always attempted to be backward compatible, so there are preferred and not preferred options still presented to you, and it took a while to figure out which tool within the program was the more current. The program also offers a variety of ways to do the same thing, so that could be confusing. Finally, the process of modifying 3dsmax to create gmax, created bugs that were never fixed...so the gmax user needs to learn those bugs exist and how to avoid them. The most obvious one is mirror tool from the ribbon is unreliable, but mirror modifier on the stack works as you expect it to.
Certain actions I'd see made no sense, like "collapsing the stack". I've seen some debate on this, and a lot of incorrect reasoning being used. Still, whether your stack is collapsed at export time or not, it will work either way.
A question I asked early on, why is there both editable poly mode and editable mesh mode? Gee, they look nearly identical. Well, suffice to say that Editable Poly is more modern, and contains a very valuable sub-object mode (border) that editable mesh does not. Later I learned that if a perfectly good mesh won't accept a weld operation, sometimes converting to editable mesh and back to editable poly will fix it. Sheesh! But I never work in Editable Mesh.
As others have said, this is more for your terminal building goals than for your ground poly goals. Still, Gmax can do both, but there are other (more modern) tools than gmax for ground polys. For 3dobjects like buildings, you can use Gmax for free. By learning it, you are learning the program interface of 3dsmax also. If you imagine using 3dsmax in the future, learning gmax now does makes sense.
Give yourself time to be baffled. That's not bad, in my mind. Just keep at it, and it will click. People on this forum give of their knowledge freely and patiently. All it takes to enjoy the skills of masters is to really work at learning it.
There are people who desired more features than Gmax provides, or simply never wanted to learn the max interface. For those folks, Blender represents a valid options. This is open source tool that serves similar markets as 3dsmax. The interface is quite different from 3dsmax. As a max user now for over 15 years, Blender is completely mysterious to me, but of course, Blender users probably feel that way about 3dsmax.