Installing/Getting started:

Drop the CopyGunMod folder into gmod/mods, then place cg_saves into your gmod folder. Start Gmod.
In-game, open the main spawn menu (Q key) and click on the 'Group Copy' button near the bottom.
In the context panel, click 'Open Main Script', then 'Give to Self'. You only have to click 'Open Main Script' once per session.


How to prepare props for copy:

When you recieve the Group-Copy Gun, you will also automatically recieve the Group-Copy Weld gun; they're both accessible from weapon slot 6. You can begin copying by using primary-fire to select a group of physics props that you wish to copy. It's important that the first prop you select is at the very bottom.

Once you've selected all the props, you can add welds if you wish.
To do this, select the Group-Copy Weld tool. Then, simply weld the selected parts of your pattern together. It should work just like the regular weld tool. Keep in mind that you'll only able to weld props that have already been selected for copy. (Such props will be colored green.)

Once you've selected and welded a pattern to your liking, you can save it to file or spawn a copy of it.


Spawning:

Spawning couldn't be easier. If you've selected/welded a set of props or loaded a pattern from file, a semi-transparent 'ghost' of your pattern will appear at your crosshairs, much like the preview for easy weld or the wheel gun. To place a copy, simply right-click. There may be some delay before it spawns, this is normal. If you're on a multiplayer server and you try to spawn something complex, then you may experience a 5-10 second delay or perhaps encounter a 'Connection Problem' error. If the latter occurs, you may be able to resolve it by opening your console and entering 'retry'. 

If you don't want the network-intensive ghost preview to slow things down, then say ('y' key) '-cgpreview 0' to disable it.
If you want to rotate the pattern before spawning it, simply say '-cgrot <degrees>'. Angles greater than -360 and less than +360 are permitted; positive angles go counter-clockwise.

If you want your props to spawn unfrozen for any reason, simply say '-cgfrozen 0'.

Saving:

To save a pattern to file, say '-cgsave <name>'. You don't need to provide a directory path or add a '.cgs' extension to the name. Pattern files are saved to gmod/cg_saves by defualt.
There's something important about saving that you have to keep in mind, though. In single-player, your patterns will be saved to the cg_saves folder on your hard drive. The same will happen if you make a save while you're hosting your own multiplayer server. However, if you join someone else's server as a client, then any saves you make will be saved to the host's drive.


Loading:

As of 1.04, there is a convenient spawn menu available for loading. Simply go to the spawn menu drop-down list and select 'Copy Gun Patterns'. You should be greeted with a clickable list of available patterns.

For an alternate method of loading, say '-cgload<name>'. You don't need to provide a directory path or add a '.cgs' extension to the name. Immediately after you enter the load command, you may experience a delay as the new prop models are precached. Once the pattern has loaded you can spawn a copy by right-clicking.
Also, the same rules for saving apply to loading. If you're a client on someone else's server, then you can only load the patterns on the host's drive.

Clearing:
To clear the current pattern, simply hit the Reload key. The preview of the current pattern (if enabled) should disappear and an on-screen message should confirm that the pattern has been cleared.




Well, that's all I can tell you about the Group-Copy Gun. Go out and try it; have fun with it!