First, Slackware officially dropped Gnome as a desktop environment.
There are others who build it, but if you look at the forums, there
are always problems. Use one of the desktop environments or window
managers that come with Slackware and you will be okay.
If your monitor is capable of 16:9 or 16:10, for instance, then you
probably don't have the proper video driver or configuration. You should
post complete information in the Slackware forum. I'm sure some wise old
Slacker can get you going. Also, you can search Google for your particular
laptop. Check for it in our HCL
, and online at Tuxmobile