Make sure you have these guidelines correct by comparing your settings with this image:
If it is all right and it still displays only wireframe, then try to enable ATI bug workaround, which should fix bugy render of ATI graphic cards. Give feedback after check.
I just dowloaded GTKRadiant from the website, and yes I have it!
And I also tried everything you said, but still the same
Maybe someone has a link to an older version?
At GtkRadiant websites are links to older versions, select one from 1.4 or 1.5, if 1.6 doesn't fit your system settings.
Make sure you have installed the latest version of 1.6 (build date 13th December 2013; correct me if there is newer version) and make sure you have all the settings like on the picture I sent you. If the bug still doesn't come out, check out misc settings in preferences and try to play with ATI workaround ckeckbox (it might fix the buggy drivers from ATI, if you have another graphics card, place the description here, you can find it in Drivers Explorer, or what is drivers administration window called in english - I have czech Windows installed)
Make sure you have really installed Visual C++ redistibutable, it's not in basic installation of Windows. Download it from Microsoft sites, link can be found on Radiant websites. Select version depending on your Windows (if you have Windows 8 32bit, download x86 version, if you have Windows 8 64bit, download x64 version). Note that you should have both Radiant version 32bit and eighter 64bit if you're running 64bit copy of your Windows.
I'm pretty sure that older version won't work well with Windows 8 since this is pretty new Windows and neighter Radiant 1.4 or 1.5 will recognise the new Windows since Win8 was created just after 1.5 has been last released. So I'd recommend you the latest 1.6.4 version of build date 13th December 2013 and Visual C++ redistibutable.
Also make sure you run Radiant as administrator, it might be sometimes buggy when running without admin permissions. Make sure Radiant folders and game folders are readable AND writeable. This means that special permission would not be the only one enabled, unlike the others that should be enabled rather than the special permission.
In general, you might misseen something or just made any mistake when checking preferences.