Have you installed all the bug fixes?

This sequence of commands works for me fine. Note that the GUAVA package is
not fully debugged for GAP 4.1. I haven't run into any issues with it yet.

From: John A Ryan <jar@sdnp.org.mw>
Reply-To: GAP Forum <GAP-Forum-Reply@dcs.st-and.ac.uk>
Date: Mon, 07 Feb 2000 20:11:54 -0000
To: Multiple recipients of list <GAP-Forum@dcs.st-and.ac.uk>
Subject: GoppaCode command

x:=Indeterminate(GF(2),"x");
G:=x^2+x+1;
L:=Elements(GF(8));
G:=GoppaCode(G,L);