Indeed, I've been hosting a Gmod9 and now 10 server for over a year and it's not all that easy compared to some other servers. The main issue is that the default install is very basic, you need to install many mods to get to the point where your server has some decent admin tools, anti-spam scripts, prop protection etc, not to mention the other mods to make your server more fun to play on, as that is the whole point of it.
To host the server you'd first of course download the dedicated server application
here, download Gmod and HL2MP and optionally CSS/TF2 etc for added media and spend ages changing settings in 50 config files. Also remember that installing mods isn't always so fun because everyone one of course uses a slightly different method to allow you to acquire them, be it SVN, some standard http download, getting it from someone's forums etc and you have to make sure it's either been designed to work or has been tested for the current server/client version. Even scripts that have been designed for the current version can be very dodgy, and if you get some nice weapons with fancy effects it may seem like a clever idea to allow non-admins to use them but this isn't always the case!
If you also want a more unique server then expect to learn/know lua or someone who does, as it can really make a server more unique if you have your own scripts. So just make sure you don't give players nukes and you should be fine, and Mr Makealotofsmoke wasn't joking about the bandwidth, with around 7-8 players you can often see uploads of 2-3+Mb/s, and that's without spamming on a build server. Same for the CPU, with lots of moving props and ragdolls the processing power used is quite high, though I never noticed the RAM usage going very high.
