Hi all!
I finally got gmplayer to work fine in my red hat 8.0!! yeah.. cool
Since this forum helped me a lot, I'd like to give you a piece of info on how to install it.
I'm not a prof, but I hope you find this info useful.

step 1: download the mplayer, a skin (I chose the blue one), a font package and the win32 codecs from http://mplayerhq.hu.
you may need the divx codecs too (do you?). Go to www.divx.com and download for linux. (divx4linux501)

step2: Installation

type:
./configure --help to see the options for the configure command.
it is clear that you should use the command:
./configure --enable-gui

then you type
make
make install

step3: fonts skin codecs
- you unzip your fonts (choose one, for example the iso 14-8859-1) to the directory /usr/local/share/mplayer/font

- unzip the skin files to /usr/local/share/mplayer/Skin
this will create a subfolder in Skin and will look like this:
/usr/local/share/mplayer/Skin/"the name of the subfolder"/
and in there will be the skin files.
go to ~/.mplayer/gui.conf and change the line
gui_skin="default" to gui_skin="the name of the subfolder"

-unzip the win32 codec files to /usr/lib/win32
also install the divx codecs from the file you downloaded.
(type sh install.sh)

step4: now recompile for mplayer to be aware of the new codecs. (repeat the 2nd step) Each time you add codecs you should recompile. (You may ask yourselves why didn't we install the codecs before the mplayer. well, I ask myself too..) (we laugh here)

step5: type gmplayer
the program should open. If not.... you may get the error about the video output (-vo) like i did.
if you try the command
gmplayer -vo x11 it may work, but still the quality will be poor.
when I tried gmplayer -vo xv got nothing.
the command xvinfo returned me that there was no adaptor (something like that anyway).
Finally, it turned out that the problem was that there were no drivers installed for my graphics card in linux.(I have an NVIDIA GEFORCE 4)
Be sure that you have your card and it's modules installed!

So what I did:
went to the nvidia web site and downloaded the driver files for linux. Then followed the installation instructions (don't miss them!)
and SUCCESS!
now in xvinfo i see a lot information about adaptors and stuff.
and gmplayer works without any -vo extensions and shit.

AHA! I got all of it by myselt except for the subtitle/font installation...nice tut

btw, you only need the divX codecs for stuff like dvds AFAIK