On FilePlanet:
"The demonstration of Half-Life 2 was unquestionably one of the hits of this year's E3. FilePlanet Subscribers now have an opportunity to experience this demo thanks to a +500 MB, 25 minute, insanely hi-res QuickTime video."

Mirrors:

http://avenger.ipaska.net/dw1/half_life_2_full_gsi.zip
http://falcon.ipaska.net/dw/half_life_2_full_gsi.zip
http://hellcat.ipaska.net/dw/half_life_2_full_gsi.zip
http://spitfire.ipaska.com/dw/half_life_2_full_gsi.zip
http://static.hugi.is/games/hl/half_life_2_full_gsi.zip
http://www.futagollc.com/genmay/half_life_2_full_gsi.zip
http://www.gutin.com/gm/half_life_2_full_gsi.zip
http://www.nova-host.net/genmay/half_life_2_full_gsi.zip
ftp://ftp.skynet.be/games/misc/half_life_2_full_gsi.zip

update:
if you haven't seen it yet, fileplanet made it public:
fileplanet
and you can also get it through BitTorrent if you don't want to wait in line for a ftp slot on fileplanet.

By netrex:
"This is the BitTorrent link I found it on:
http://www.aixgaming.com/torrents/half_life_2...ip.torrent
BitTorrent:
http://bitconjurer.org/BitTorrent/download.html "


hf :)