I can find a statement at the beginning of the user's manual:
version 3 for Linux
(tested with Debian and CentOS 32-bit and 64-bit)
I am using Ubuntu 12.10 64-bit for development.
So far, I found no problem with it, but I still have no confidence on it, since there is no official announcement from gwan about this.
At least G-WAN v3.3 should be compatible with the Linux distributions used when the development started in 2009 (Ubuntu 8+).
G-WAN v3.10+ added significant enhancements detailled below.
There are many different Linux distributions, but most of them use a common 'root' distribution, and the most commonly used are Debian (Ubuntu uses Debian) and Red Hat (CentOS uses Red Hat), both of which have been tested during development.
But the other distributions rely on the same Linux kernel, making any difference only possible in 'extensions' like file systems, etc. And, thanks to its users feedback, G-WAN v3.10+ has fixed non-standard file system issues (by handling those FSs that ignore system flags).
For this same reason, old GBLIC versions could also a problem for G-WAN v3.3, but G-WAN v3.10+ re-implemented those function calls that could pose problem so 10-year old Linux distribution should work fine now.
This is why list of Linux distributions that support G-WAN is very large - probably too large to be worth publishing.
Now, the kernel question is more interesting, not because it is a blocking issue (in G-WAN v3.10+, wrappers have been written for potentially missing syscalls since the Linux kernel 2.5.8) but because of the lack of performance on very old Linux kernels.
In those old kernels, G-WAN can't benefit as well as multicore systems because the OS kernel itself does not support multicore. But since G-WAN is fast server even when used with one single thread, G-WAN will not be the bottleneck.
Compatibility is an incremental process since new versions of the system parts (OS kernel, GLIBC, other system tools and libraries, etc.) are not always backward-compliant (remember the recent Linux linker issue).
In this regard, user feedback rules - and G-WAN has made tremendous efforts in this matter recently.