Under Linux, there are basically two ways to adress GPIB hardware:
As much of the hardware is bought from National Instruments, one is the proprietary National Instruments VISA driver stack on top of NI's GPIB hardware driver. This works very reliable (once it's installed), supports e.g. LabView for Linux (eurgh), and NI is actively developing drivers for (some of its) hardware. So much for the good sides. Here are it's disadvantages:
- installation is basically doing its own package management, only very few (mostly outdated) linux distributions are really supported and trying others can be a serious pain, and the entire installation process is fundamentally incompatible with portage (I've been trying to beat it into an ebuild for a while and this is a sure way to madness);
- since the Linux kernel USB interface is declared GPL-only, no GPIB-USB devices are supported since kernel 2.6.24.
As a final remark, probably the best possible support would be to combine the open-source linux-gpib hardware driver with the NI VISA intermediate layer. Unfortunately, this would mean linking a GPL library and proprietary code, so no-go. At least as long as nobody budges...