You'll get much better package search results using a program called
Synaptic, which is available in the software center, which you may never
use again after seeing synaptic. I recommend not installing any of the
Ubuntu packages, and obtaining the packages from the maintainer or via
quicklisp. This includes SBCL and slime/swank especially. The main issue
is that Ubuntu tends to freeze a version a few months in advance of
release, and some very good improvements are being made to the software,
which take perhaps 6-10 months to arrive in a default Ubuntu repository.
If I recall correctly, 'GCL' is CTLT1, not Ansi CL, so it is likely
there will be incompatibilities. GCL exists to provide support for some
older programs. clisp, as pointed out, is a different program, and
perfectly suitable.
Firing up clisp 2.49 on my Ubuntu machine, I find that *load-pathname*
is defined, and is nil. That looks like the correct behavior:
http://www.lispworks.com/documentation/HyperSpec/Body/v_ld_pns.htm
Loading gcl-2.6.7, I agree that *load-pathname* is indeed unbound. Is
that a CLTL1 feature?