Gnotella does use quite a bit of memory. I have an app called MemTurbo that I use to periodically defrag and reclaim the RAM that Gnotella uses. Also, when you close Gnotella after extended use it will take a while for it to actually close and return the RAM to the system. You can really close it by doing a ctrl-alt-del and killing Gnotella if the ctrl-alt-del window shows "Gnotella(not respomding)". This immediately returns the RAM to the system. The current version of Gnotella(0.73) is much better than previous versions when it comes to this RAM issue.
I use Gnotella all the time and have downloaded and uploaded hundreds of files with it. It's a great program and it gets better with each release. The ability to limit incoming connections with Gnotella is essential when you are on a 56K dial-up like me.