This is essentially the last of the GURPS in a nutshell posts.
Experience and character advancement in GURPS is even more
theoretically simple and practially complex as GURPS character
generation. Characters in GURPS receive additional character points,
awarded by the GM. These can be spent as character points are spent
during character generation. However, gamemasters are encouraged to
restrict this expenditure. If the only thing that characters do is
wander through a jungle and hack monsters in half with axes, then the
only skills they can acquire via experience character points are
Survival (Jungle) and Axe. Of course, in most campaigns, characters
do a lot more than this.
In addition, gamemasters may allow a character to "study". If they
can find a teacher, then X hours equal 1 character point in the skill
studied. If there is no teacher but there are adequate texts,
practice facilities, etc., then this rate is half as effective. If
there is no teacher and no adequate other source of information, then
the rate is one quarter as efficient. Some skills may require
literacy to study.
Gamemasters can award "free" skill points, too. For example, if
Intrepid Max and his flunkies are wandering around France for six
months, they might each get a "free" character point to pick up or
improve French (up to a reasonable limit).
Character points can be used to improve attributes, but usually at
double cost from character generation.
Character points cannot usually be used to pick up new Advantages,
although a gamemaster may make exceptions.
Character points may be used to eliminate Disadvantages, but only with
gamemaster permission--and gamemasters are encouraged to incorporate
this into active roleplay sessions.
It is possible for characters to acquire some Advantages due to
roleplaying events. While GURPS encourages gamemasters to charge
character points for these, many gamemasters ignore this suggestion,
using the principle of "earned in play is earned".
Characters can get new Disadvantages during play. These disadvantages
never (as in NEVER) grant more character points. All GURPS
gamemasters I've encountered adhere to this.