Too fucking stupid to install Pirate Wars?
We can help! Well, the installation anyway. You'll still be a dumbass afterward.

Right click the file and choose some manner of extraction.
This will be ALZip, 7-Zip, WinRAR, or something similar.
If you have no such option, double-click (XP/Vista) to bring up a built-in but much slower extractor.

STEP TWO: COPY THE ADDONS, MORON!

Navigate to the folder you just extracted. Now, browse to the addons folder contained there.
Right-click the bg2models folder and click 'Copy'. Now, go to your Garry's Mod directory.
Right-click on the addons folder in your Garry's Mod directory and click 'Paste'.
Click 'Yes' on any warning dialogues.

STEP THREE: COPY THE GAMEMODE, RETARD!

Navigate to the folder you extracted. Browse to the gamemodes folder and right-click/copy the sparky_piratewars folder.
Now, in your Garry's Mod directory again, paste it into the Gamemodes folder, again clicking 'Yes' on warnings.

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That's it, you're done. Grab a beer, man! If you didn't fuck up, you can now connect and play Pirate Wars.
If not, you're more of a retard than this guide can help.