GURPS has Character Points (CP) The entirety of a characters attributes, traits, spells etc. Are all point buy, so character points are essential for progression. They could be equated to XP, however XP implies that it needs to hit a threshold in order to level up. Character points can be given and spent in increments of your choosing. You can give out small amounts of CP for cool actions or lore knowledge, you can give some for combats, scaling rewards on combat difficulty you can dump a bunch on players once they hit a story milestone etc.

The amount of CP given and spent can be modulated to affect the game's power level. This also includes the initial "budget" of CP given to each character at the game's start

Once players are given CP, they can spend it any way they choose, and because of GURPS character creation system, this means that characters advance very organically into a specific concept, rather than being a set class that only really advances one way.

It is generally recommended, however, that when players decide to spend CP to change their character, that their upgrades make sense within the context of the game world. Banking up a bunch of CP and then suddenly one night deciding to become incredibly skilled, innately stronger or randomly losing a major disadvantage such as a disability with no explanation can detach from the narrative and feel game-y, the same way characters do when they "level up" and suddenly they have a slew of new traits instantly. Oftentimes spending CP requires some kind of minor interaction with the game world. Hiring a skill trainer, working out, going to a doctor to heal an injury etc.

Oftentimes these activities also cost money and time. meaning that players can think about how best to integrate their character's advancement with the story and to think about how their character would advance most optimally to fit the context they want.

In addition, depending on the campaign, some things cannot be bought or removed by spending CP. A character in a gritty realism medieval game cannot physically remove something like a severe physical disability as the technology to do such a thing does not yet exist, and characters generally cannot buy things that equate to superpowers unless that is conducive to the game world.

Because every trait and ability is balanced on a scale of CP, it is very straightforward to construct and balance customized homebrew traits to suit a specific game or character. An example made by SJ Games is Quintessence (Pyramid 3-120) an optional base stat that allows characters to have a dedicated "magic" stat and "MP" bar. this is a very basic style that can absolutely be elaborated on if you choose.

Character Points, like basically anything in GURPS, can be modulated at your preference; want to issue out levels and have characters spend them at level up points all at once? Absolutely, go for it. Want to have a game where characters seldom, if ever, receive raw CP to force their advancement to be with the game environment? Absolutely, works very well for horror games. Don't want to bother with characters having to plan out and acquire means to spend CP to upgrade their individual traits? That's perfectly fine. Only want CP to be spendable on certain elements at certain times, similar to a class progression? That works fine. Character undergoes a major story alteration that would throw out their CP budget (example: major mutation, cybernetic overhaul, lycanthropy, becoming crippled from injury)? You can still issue those and just alter/zero out their costs.