You can extract *.BSP files from *.GCF files using Nem's GCF Scape. You can also use Nem's Crafty to view maps (if that's what you're asking for). If you realy want them in 3DSMax, you need to decompile them, and open/save them as *.DXF files using Valve's Hammer Editor.
You need a decompiler (bsp2map, winbspc, etc.) to decompile *.BSP files, and convert them into *.MAP files. Remember, you can never get original brush/or other data by using this way, still it's good enough to use as a base in 3DSMax.
What do you mean "that's what I get?" Am I doing something wrong? Anyway I just wanted to make some maps for Halo Custom Edition using Half Life maps. Not 100% legal, but I won't get sued or anything since it's not for profit.
Nope, it's just about decompile operation. It's not totally accurate, not to mention amount of invalid brushes. Whatever, good luck. Valve doesn't really care about HL content, so I guess you're free to use anything you want.