In brief, you cannot compile GAP3 directly on a 64 bit machine. The 32 bit
assumption is wired into the storage manager, long integer arithmetic and
Blist code. You can download an alpha binary from Aachen (it was compiled on
an older DEC workstation and converted using a DEC conversion tool).
A new patch level of GAP 3.4 (3.4.4) will be out in the next week or two. It
rolls in the 30 or so bug fixes since 3.4.3, makes a few tiny improvements and
picks up the latest versions of all the share packages.
GAP4 will be out this summer and is a complete re-engineering of the kernel
and the library structure. You can get an alpha test version now if you are
interested, contact email@example.com. It will run on 64 bit machines
when it is released (but we haven't quite integrated the code for this yet).