if you own GTA4 and buy the GTA complete pack, what happens to the extra copy?

I know steam has sort of complicated rules regarding extra copies. Would I be able to gift it in this particular case, or no?

 

You should be able to gift it like any other game.

 

Know ahead that it is not giftable, unless they expressly state otherwise. It simply disappears into the ether.

 

According to this,

Extra copies will only be issued as a special promotion in the following cases:

Users who own "Left 4 Dead" and then go on to purchase "Left 4 Dead + Left 4 Dead 2 Bundle" will receive an extra copy of "Left 4 Dead"
Users who own “Half-Life 2” and go on to purchase “The Orange Box” or the "Valve Complete Pack" will receive an extra copy of “Half-Life 2”.
Users who own “Half-Life 2: Episode One” and then go on to purchase the "Half-Life 2: Episode Pack”, “Valve Complete Pack” or “The Orange Box” will receive an extra copy of “Half-Life 2: Episode One”.
Users who own "Dawn of War II" and then go on to purchase "Dawn of War II Gold Edition" will receive an extra copy of "Dawn of War II"
Users who own "Titan Quest" and/or "Immortal Throne" and go on to pre-purchase "Darksiders" will receive an extra copy of "Titan Quest" and/or "Immortal Throne"
Users who purchase the available four packs for specific Steam titles will receive three extra copies of that title to gift to friends.

 

You also get an extra giftable CoD4 if you pre-order MW3 and already own CoD4...but as Kevin said, if you'll get an extra copy of a game, it will state that clearly ahead of time. Otherwise, assume no extra copy.