Yes, garry's mod will work with portal and you will have all the HL2 stuff.
Make sure you have portal installed on steam, then start up garrys mod, and go to Extensions, switch to the game content tab on the right and put a checkmark in next to "Portal". Restart Garrys mod and you will have the content for Portal available.
There are no cheat codes in Garry's mod as it already contains a sandbox mod where you can fly around and spawn props. But I know there are some commands you can insert into the command console. Such as sm_cheats_1 which enables third person so If I were you do a little poking around to find some stuff like that.
You do not have the correct addons needed for those props. That's why there is an error, get used to the names of addons, then download and install them from garrysmod.org
In general you only get props in Garry's Mod if they exist on your system, or in other words if you have that particular Source game installed. Whilst Portal might contain some generic Source props (and Garry's Mod may supply some itself) it's entirely likely that some (if not most) of the HL2 props will be missing if you don't have HL2
No. Unless it's a glitch, the only way you can spawn them other then mods or console is going into the NPCs section and manually spawning a zombie.
Buy Garry's mod 10 off of steam.
No, GMod (Garrys Mod) is a sandbox physics game where you can build what ever you want and can using props from many games by Valve
Garry's Mod 10
technically yes, there were a few updates it is still called Garrys mod 10 though
In order for Garry's Mod to work you need a minimum of one Source Engine game eg. Half-Life 2, Team Fortress 2 or Portal.
no because garrys mod requires that you have a game that uses the source engine