Looking at GURPS Streetwise

 Streetwise is one of the influence skills, meaning it can be used to replace a reaction roll in the appropriate circumstances. It is an Average skill, typical of them, though Carousing is Easy and Diplomacy is Hard, the rest are all Average skills.

What do the rules say its used for?

GURPS Basic, p. 223 says its the skill to get along in rough company and "You may substitute an Influence roll against Streetwise for any reaction roll made in an underworld or “bad neighborhood”  situation; see Influence Rolls (p. 359)."

GURPS Social Engineering, p. 30 says "Streetwise isn’t the same as Intimidation; the latter makes you seem dangerous and hostile, while the former conveys that you know your way around. "

This can be a bit risky as a failed Influence roll (p. B359) can get you a worse reaction than not using an influence skill, and the roll is resisted by Will. So if you are going to rely on it its a good idea to be very good at it.

Additionally the skill lets you ask for information among criminals and seeming like you belong there rather than are an undercover enforcement officer or perhaps a criminal rival. Specifically it helps you find the criminal element, or those catering to it (such as corrupt police), know a good bribe price, and perhaps find the local "action".

It is not used to buy things (use Merchant) but at least it can help you seem like a legitimate customer and perhaps find venders of illegal wares. Also Streetwise at 12+ cancels the normal -3 to Merchant for buying illegal goods.

Overall a decent skill, but can be risky to use and perhaps lacks flavor and oomph for that cinematic expert. So here are some additional suggestions.

1) Allow Optional Specialization (p. B169) for certain types of criminal elements. Examples Docks/Ports; Gangs; Neighborhoods; etc.

2) Add techniques such as...

Fits In (H) Streetwise-3, purchased up to Streetwise (at 4 points) to offset the penalty for obviously being a stranger in the area.

Cutting Out (H) Streetwise-4 to pick an individual out of a group to use your skill on. Social Engineering, p. 81)

Hinting (H) Streetwise+0 (up to Streetwise +6) for being subtle enough in your questions that observers might not pick up on it. (Social Engineering, p. 81).

I really like this one as its often a good idea to be discreet when asking about illegal activities!

3) Certain perks and other advantages can of course also help. "In the Know" (Social Engineering, p. 79) grants +2 to finding an illegal connection, such as a fence.