If you open your setup.ini (with notepad) and change this line to : "disabledynamicres = 1" your GG will always run in your native monitor resolution.
"High Definition" is a term that somehow wormed itself into the game development scene but in essence, as Wizard has already pointed out, its mostly related to monitors and image resolution.
Good graphics don't come from settings. They are an intricate mixture of technology and art and are thus very hard to craft. While good rendering and shaders greatly influence how your game will look and feel, what makes it "look good" will be the models and media you craft/use. Making models as they are seen in todays videogames is a tough skill to master and it takes years to do so. Lighting is a key factor in this too.
A lot of "HD" mods for games replace textures with higher resolution ones, I believe this is where the term came from and alas...its a good buzzword to market things (The HD rerelease!)...this can result in an improvement in visual quality but does not need to. The texture itself is a bit more important than its definition here. Even now you'd be surprised how Low-Res a lot of content in contemporary games really is.
I suppose there is little advice I could give. Look up 3D modeling, teach yourself the basics of texturing and what the different shader maps do and maybe look at a few games you like visually and how they are made.
There are a lot of different styles and approaches to visuals in our showcase and work in progress boards...you can browse these to get an idea how people achieve the graphics of their games in GG.
-Wolf