You could try taking the soundscape files from the "GameRoot/gamefolder/scripts" out of the GCFs of the different Source games, drop them into the "garrysmod/garrysmod/scripts" or into a custom folder there like "soundscapes".
Then make a custom .TXT file named soundscapes_manifest.txt in the "garrysmod/garrysmod/scripts" directory and add those soundscape text files using the
Code:
"file" "scripts/soundscapes/soundscape_%notes%.txt"
Doing so allows you to precache the soundscapes without having the games mounted, which is based on the game_sounds_manifest.txt that has the soundscripts from the games precached