Learn English – reason for the “9” in teen slang “code 9”

This Wikipedia article gives this explanation for the origin of the word gee-gee:

The Chester Racecourse site was home to the famous and bloody Goteddsday football match. The game was very violent and, in 1533, banned by the city, to be replaced in 1539 by horse racing. The first recorded race was held on February 9, 1539 with the consent of the Mayor Henry Gee, whose name led to the use of the term "gee-gee" for horses.[1]

[1] a b Marcus Armytage (2008-05-06). "Chester racecourse moves with the times". The Telegraph. Retrieved 2008-05-09.

This article expands on that:

The founding father of Chester Races, Henry Gee, was a zealous reformer who is said to have put the corporation house in order "With a high hand and an unswerving purpose". He suppressed corrupt municipal practices and appointments (where is he now, we wonder?), banished "Idle beggars and vagabonds", regulated the markets and established the first attempt at a school board. He banned single women from keeping common ale-houses and "Stamped on immorality wherever he found it". Henry Gee died in 1545, but his name is remembered in the running of the Henry Gee Stakes for three-year-old maidens at the July meeting, and possibly also in the old, but still commonly-used, English nickname for racehorses: Gee-Gees.

This definition of gee-gee says it is a UK child's word for horse. I don't believe that is used by children in the U.S.

I don't know if this is relevant, but gee or gee up is also a command to get a horse to move faster. (It is also used to have a draft animal turn right. As opposed to haw, which is a command to turn to the left.)

From "Horse racing jokes" on http://www.ukjockey.com/jokes.html

How do you spell 'Hungry Horse' in four letters?

M T G G (Empty Gee-Gee).

As FumbleFingers says, this is a pretty old use, going back to Early Modern English:

Pretty (pri•ti) adv. Forms: see prec. [The adj. in adverbial use.]
 1. To a considerable extent, considerably; in a fair or moderate degree, fairly, moderately, tolerably; [...]
1598 FLORIO Dict. Ep. Ded. 3 Boccace is prettie hard, yet understood: Petrarche harder but explained. —OED 1

(I offer this OED 1 citation rather than the earlier one reproduced by FumbleFingers because I am not convinced that the use in the earlier citation is adverbial.)

Pretty has a complicated history, as you may see from the OED 1 entry which immediately precedes the link above: from cunning, artful to clever, skilful to ‘a general epithet of admiration or appreciation... fine, pleasing, proper’, and so forth, leading to the main modern sense ‘having beauty without majesty or stateliness’; but the origin of this adverbial use clearly lies in OED 1 sense 5 for the adjective:

 5. Considerable in number, quantity, or extent, as in a pretty deal, while, way, etc.; also a pretty many = a good many; ...

OED 1 gives citations for this use dating from c. 1485 down to 1861, but marks it as ‘Now arch. or dial.’

I am surprised you did not find this in any dictionary; I found it in the first three online dictionaries I checked, Collins, Oxford, and Merriam-Webster. All three post definitions for the adverb after those for the adjective.