iago helped me set up slapt-get the other day, and I needed to get Gnome from it so I typed:
"slapt-get --show Gnome"
and nothing came up. I tried varying capital letters and stuff in Gnome too, and it didn't work. I'm currently using XFCE because KDE owns my beast of a PC (400MHz, 384mb RAM, 2mb video card) and I want to see why people seem to like Gnome as much as they do..
How do I get Gnome?
Wow, yuck. Get a new PC.
Anyway, I wasn't aware that you'd use slapt-get to update individual packages. Personally, I'd just download the Gnome packages using wget and install them manually. I just use SSH on my server though, so I don't have too much experience setting up/using X.
There's nothing wrong with that computer.
XFCE is similar to Gnome, except lighter.
Try using --search. --show only works if you have the name exactly right, but it's probably not called just gnome. And BTW, make sure you've run --update and are pointed at some mirrors.
UPDATE (not edit): I tried --search, and I don't actually see it. I know Slackware was going to stop supporting it, so maybe it got lost in time on the various servers. It sucks anyway, XFCE is much better, and similar looking.