I've got TF2 and L4D, and on another Steam account I have HL2 and Garry's mod. Is it possible to consolidate them in any way so that I can access the TF2 and L4D content on the account that has HL2 and Garry's mod? Thanks. 

I don't think so.  The Steamapps folder is directly below the ClientAccountName folder and AFIAK Source Apps all rely on local directory structure.  You might want to try and copy the content from the account folder which has TF2 and L4D (ie the extracted game folders themselves, not the GCFs) into your the account folder which has Garry's Mod, but I can't guarantee that it would work.  The other options are either to try and see if Gmod9 (which is free) will do enough for you or to purchase Gmod10 again on the account that has TF2 and L4D.