You definitely CAN use Gnubox on 5500, i'm using myself, so my 5500 connects to the internet via bluetooth to pc:
5500 -> bluetooth -> pc -> adsl -> internet
Gnubox can be installed on 5500, it is only a long way to go: i found months ago a guide on how to download the unsigned version of gnubox for s60v3 and to sign yourself with a windows program. If the url don't get removed, it is here:
http://discussion.forum.nokia.com/forum/showthread.php?t=96953&highlight=5500+gnubox
Otherwise, just search 5500 gnubox on discussion.forum.nokia, the discussion forum on nokia site (not europe!).
It is tricky, but it works. You have to setup your pc, so a gnubox site somewhere should help you do that. The only hint I can give you is that the Bt connection should be setup as an high speed connection and not as gprs. I had to give some tries before making it working, but eventually i ended up with the 5500 connecting to internet via pc. After that, i just setup pc to let it accept connection (i use Linux, btw), pairing is needed obviously. Then gnubox -> debug -> "bring up if". It signals "connection open" and works like a charm.