To my surprise there apparently hasn't been support for bash command 
line completion for the gem command. I've cobbled together a file that 
add just that. See

http://schuerig.de/michael/blog/index.php/2007/02/24/gem-bash-completion/

I've only tested it on Debian/Linux, on other Linux/Unix variants things 
might work somewhat differently. Also, I don't even try to do 
completion for gem names; doing that for remote gems would be much to 
slow and I haven't looked into getting that information from the local 
source_cache.

Michael

-- 
Michael Schuerig
mailto:michael / schuerig.de
http://www.schuerig.de/michael/