Mechanics: Metagame

Not this kind, and I don't even want to
know what it is.

Metagame. It is a supremely overloaded word, that has definitions that place it anywhere from subjectively bad, to objectively neutral, to subjectively good, so before I begin this post, I need to make sure we are singing from the hymn sheet.
Today, when I say metagame, I speak to mechanics that have an impact on the game via the interface of player and game, rather than character and game. By that I mean... normally, the player interacts with the GM's world by speaking and working through his character. The player can therefore only do something that his character can do. Metagame mechanics subvert this usual contract, for better or for worse, to the benefit or detriment of the player by allowing the player some sort of agency that doesn't come from his character. GURPS doesn't have as many metagame mechanics built in as a lot of other systems, but there are still some, and even among those, some are so important, major elements are predicated and built upon them as a foundation. Let's investigate further.

Advantages

Among the advantages in the basic set, the following enable meta-game mechanics.

• Destiny (p.49) Destiny at its various levels is some form of insurance that your character will live long enough to accomplish something important. This shouldn't be abused though; an example given in the description text says throwing yourself on an assassin's knife can still kill you... and then fulfill your destiny of dying a hero.
• Luck (p.66) Luck allows a player to retcon a failure when success of a particular roll is imperative. This advantage is often recommended because it is pretty affordable and acts as insurance against random bad stuff.
• Serendipity (p.89) An analog to Luck, where luck rewrites the story so a bad thing didn't happen, Serendipity is about rewriting the situation so a good thing did happen. It allows players to suggest a particular lucky turn of events, but the GM ultimately decides if it comes true.
• Signature Gear (p.85) Signature Gear is plot protection for items that are critical to your character. Items designated as signature gear that are lost can always be retrieved through reasonable effort.
• Super Luck (p.89) similar, but more powerful than regular Luck. It guarantees success at a particular dice roll no matter what.

Disadvantages

Several disadvantages have metagame application as well. Especially, most disadvantages with a self control roll manifest in a way that needs to be role played. For example, Truthfulness manifests as making it impossible for a player to make their character lie. Besides the self-control disadvantages, the following have meta-game effects.

• Cursed (p.129) Good things can't happen to you, and if bad things happen to the party, they will affect you instead or worse than the rest of the party. Reality warps to make your life miserable.
• Destiny (p.131) Similar to a good destiny, but bad. It's plot protection that protects your character from minor awful things just so they can fail in the most utterly spectacular way down the line. This is more like a fate worse than death, it is the inevitable combination of failing, embarrassment, and abandonment.
• Weirdness Magnet (p.161) is like the nega-version of Serendipity, where it causes inexplicable bad things to happen. It can be used to shape the plot, making bad things just happen because your character is around.

Character Development Mechanics

According to p.498, players that portray a character accurately, regardless of efficacy should be rewarded for this with character points. Solving problems in a unique way that aligns with a character's strengths and weaknesses gives the player even more character points.

Impulse Point Spending

A player can use unspent character points in a number of ways to influence the world around their character or change the outcome of a situation.

Haven't read it yet,
Just the quip in 4th Edition
Festival, but it sounds interesting.

• Unspent character points can be used to change the outcomes of a roll with rules on p. 347.
• Powerful attacks can be mitigated by Character Points if called out as Flesh Wounds (p.417)
• Those two and more methods of spending character points, metagame mechanics or otherwise are expanded on in Impulse Buys. It includes ways to turn this "burning" of character points into a permanent pool, so it doesn't have to feel like a waste if that is something that bothers you (and it is something that bothers me!)

Other Thoughts and Closing

Metagame mechanics like cooperative setting building and story telling have been in vogue for a while with new RPGs. GURPS has some mechanics for metagame functionality, but overall, a lot of those functions need to be houseruled or built into GURPS. I feel like the category of metagame mechanics that has the least support out of the box are the cooperative awards given between players, like rerolls for "doing something cool." Besides those though, most can probably be done with character-point intensive extrapolation and application of other existing mechanics in GURPS.
Edit: While doing a browse through the internet for similar and related information, I found this post that covers the same ideas, but differently.