You can use the ingame extract function or my handy dandy GK3 Browser to extract the files you want. Which files do you need? Take a look at these pages. They're divided up by character.
Note that these pages were generated by a script (you think I was going to go through each dialog file by hand? There's over 7000 of them!) I wrote, so there might be some strange stuff in there due to bugs in the script.
Also, GK3 dialog files do not specify who the speaker is if it's the current player's avatar doing the speaking, so lots of Gabe's and Grace's lines are listed as UKNOWN. Maybe someday they'll all be sorted out, but there's a WHOLE LOT of those so it will take a while.
Character line info!
More characters to come!
Disclaimer:These pages are posted solely for educational purposes. All the dialog from GK3 is owned and copyrighted by Sierra. Since movie scripts and song lyrics can be posted on the internet under Fair Use provisions, I'm going to claim the same and hope it applies to video games.