I've seen a lot of people discussing trading games on GOG.com however I've never been able to locate a thread talking about how this is done? Before I dive down the rabbit hole I'd like to know how one goes about doing such things. More specifically how we obtain our "Key". I looked around on the site and I haven't found a way. Do I have to download the games I don't want? Thanks!

Foghladha

Key is basically a gift. When you buy a game you can choose to purchase it for yourself or as a gift.

trading itself is based off trust

This is the trading thread. You can read the OP for the basic rules:
https://www.gog.com/forum/general/gogs_general_trading_thread/page1

As far as trading GOG games, you need to be sure to purchase them as gifts. You can send the gift to your own email, and you will receive a key that can be gifted.

Good luck!

There isn't an actual trading system, you just have to buy games as gifts and then trade the codes to people who want them.

There's a thread.

GOG Trading Thread:
http://www.gog.com/forum/general/gogs_general_trading_thread

Edit: Ninja'd, three times!

Thanks everyone that helps a lot :). Now I know Mutators are gifts :D
So can I use the keys on my own account?

If so then is common practice is probably to buy everything as a gift and only apply them when you decide to actually play them?

Yep, you can redeem those gifts yourself. Many people buy games from the Mutator promo as gifts and then only redeem the ones they want.

By the way, you should be aware that you will only get one gift key for the whole order, not separate keys for each game. When redeeming a key, individual games can be selected by the recipient and any remaining unredeemed games will still be associated with the key. Just something to keep in mind.

Just so I know: why do people buy giftcodes for games they don't want and them trade them for games they do want? Instead of, you know, just buy the games you want directly. Am I missing something?

Sales.
People buy gift codes for cheap during sales and try to trade them after the sale for games with a worth similar to what the un-discounted game code would have been worth.

or like promos like the mutators. You get 3 random games. Most of which you don't want.