you are running debian and you said there is no driver for your GPU ? lol
first copy the following file
XF86Config-4.conf into XF86Config-4bis.conf
or
xorg.conf into xorgbis.conf
then run command line :
#su
enter your password
#dpkg-reconfigure xserver-xfree86 or xserver-xorg if you have xorg !
Then you ll see the xserver configuration
It will certainly remind you something you ll see

(it was the configuration screen you saw when you installed debian)
choose medium when the subject is your screen. then choose your screen display.
Anyway, if there is a problem, boot with the recovery mode
then enter your root password and then
# cd /etc/X11
# vi XF86Config-4bis.conf
or
# vi xorgbis.conf
the vi command are :
a to insert text
ESC to escape insertion
: to enter a command
:save XF86Config-4.conf
or
:save xorg.conf
:q to quit
---
BTW debian shouldbe able to install your graphic card :
try
# lspci | grep vga
if there is no result or a unknow card result
try :
# update-pciids
# lspci | grep vga
if anything changed you can retry to install your GPU but really I made the mistake : everytime install debian drivers instead of internet ones : add non-free contrib to your apt sources.list