Assuming you're on a fairly standard Unix system and you are in a hurry,
you can quickly build/test GP in the following way:

a) look at MACHINES to see if anything funny applies to your configuration
b) ./Configure (--prefix=/exotic/dir/name if desired. Default is /usr/local)
c) make gp, make bench
d) make doc to build manual, tutorial and reference card (produces users.dvi,
tutorial.dvi, refcard.dvi, refcard.ps in directory doc).
e) make install if desired
f) copy misc/gprc.dft to $HOME/.gprc. Modify it to your liking (possibly copy
it to /etc/gprc for system-wide installations).
g) copy/modify misc/gpalias somewhere and call it from your .gprc (if desired)
h) the data for polgalois() function (12 > degree > 6) is provided in a
separate archive galdata.tgz, which you have to fetch separately.

That's it! Compile and read INSTALL.tex (e.g tex INSTALL; xdvi INSTALL)
for more detailed information and various hints.

P.S: the same procedure should work on
  Windows 95/98/NT: with the cygwin package
  OS/2            : with EMX, GNU tools, and pdksh