Hmmm, this is not so much on the subject of the evil things we would like to do to those who release GOG games on P2P, but rather on what we as the community could do to help prevent it. What if we were to take a page from the DRM playbook and "pollute" P2P and torrent sites with fake GOG downloads? Instead of games, we could put multiple copies of a self-executing Rick Roll video out there.

Or better - just put out the official demos presented as full versions. But TBH, the last thing we need is to give the appearance that there are lots of GOG-games on the torrent networks.

I AM FOR THIS

So if someone posts a torrent which says, for example, "Prince of persia - GOG release " it's obviously happening otherwise it just isn't? After a quick search I have found several copies of Prince of Persia. Who is to say thay they don't come from GOG?
Come on! The reason why GOG stuff isn't appearing on P2P sites is that no one wants them, the recent stuff, expensive stuff is what keeps the torrents alive.
Yes, there are times when a GOG supplied game has been copied, this "mutual trust" you have quoted does not exist, don't forget what you're involved in here. GOG, is almost negligible in relation to the rest of the gaming community.
Bigger, faster, better is the motto to keep the pennies rolling in and old games have a fanbase but it isn't exactly on a large scale. They've already been cracked moons ago and those who have wanted it already have it.

I think Zeewolf makes an excellent point: we don't want to increase the apparent numbers of GoG games on torrent sites.
What would I like to see happen to these guys? I'd love their next legitimate software purchase to be infected with a vile form of DRM, have it eat their optical drives, and have it bork their OSes, forcing a reinstall and a complete loss of data.

While the rick rolling is commendable, a far more lasting and unforgettable punishment would be preferable.
How about strapping virtual reality binoculars to their faces with superglue that fuses to their sideburns; these would show them endless hours of Teletubby reruns where they would be forced to play a role.
OR
Training an army of seagulls to follow said perpetrator and defecate on his head whenever he began pirating any GOG games, followed by immediate submersion in a tank of heavily diluted Kool-Aid (93% water), and finally release into a glass room filled with sugar-hungry giant wasps.

I gotta go with my personal favourite yet again: Trampling to death by ducks.

Ouch. That would take a reaalllyy long time to do.
My wife actually came up with this one. Skewer the bastard up the ass with an umbrella... then open it.

You know if it comes from GOG because they create a special (much easier and more compatible, might I add) install file for their games.

@cogadh Ouch, remind me never to pi** you off!

I'd like to see the pirates say "woah, this is pretty cool. I'm gunna check out their site and buy some games there".

hah you ever heard of the Pear of Anguish? much much better that an umbrella.
http://en.wikipedia.org/wiki/Pear_of_Anguish
Also
Thumbscrews

Well, there is of course the "piracy as advertising" aspect of it. Let's hope so.

Yes, you can tell from the files. Pirate releases don't come pre-installed so are usually in the form of disc images (ISO, BIN+CUE, etc.), not EXEs.