A quick inexpert answer (having no experience with Gigasampler):
Gigasampler will sound better.
Sound Fonts can sound pretty darn good, though.
I don't think I've ever seen Gigasampler sounds for free on the internet... but I haven't actually looked around for any. Sound Fonts of all qualities, from very very good to horrendous, are freely available all over the internet, and there are some very good ones you can buy, too, for a very reasonable price, comparitively.
Sound Fonts will be cheaper and easier to use overall if you have a single computer and if you have an SB Live soundcard. Having one machine running a recording application and any softsynth like Gigasampler is going to impact the resources available to the recording program, as in reduce the number of tracks you can get and/or the number of plug-in effects you can use. Of course the SB Live will eat some RAM to load up sound fonts but if you have a lot it won't affect things much. To get the best from Gigasampler you should really dedicate a computer to just running Gigasampler. This is not to suggest you can't get fantastic results running it in conjunction with a recording app on a fast PIII or higher...
Oops... almost forgot your other question... a good place to find some very good sound fonts is Hammersound:
http://www.hammersound.net/
As I mentioned above, I don't know of any such sites for Gigasampler files.
Good luck!
-AlChuck
