GURPS Skills are fairly straight forward. You start by paying 1 point which gets a skill level of your Attribute minus something. If it is an easy skill then it is minus 0, a Average skill is -1, Hard is -2, and Very Hard -3. For example Knife skill is DX/Easy, A boardsword is DX/Average, Karate is DX/Hard, and Blind Fighting is Per/Very Hard.
Subsquent skill levels have an increasing cost. You pay a total of 2 points for +1, 4 points for +2, a total of 8 points for +3 and +4 points for each +1 skill afterwards. The scale works out so that at normal points levels you can have a bunch of skills at average level and be specialized in a few.
The reasoning behind difficulty levels is spelled out in a short section of skill chapter. It basically boils down to challenging skills like Biology, Alchemy, etc being rated Very High and ordinary everyday skills like Knife, Area Knowledge, and Bicycling Easy.
This brings up another point about GURPS. It's adherence to realism. In the absence of a specific genre reason the designer GURPS justify their rule decision in terms of gamable realism. The skills difficulty is based on real world consideration not internal game balance. Alchemy is difficult to learn because it requires a lot of time and study. They don't have any thing complicated just a single chart with each level of difficulty as a column. You can see this in GURPS Lite.
Skills also have defaults. So if you never learned Broadsword as a wizard and decide to pick one up in the middle of a fight then you either default to DX-4 or if you have another weapons skill then you get a better default like Shortsword -2.
The Skill list is extensive but not excessively so. Few skills overlap and for Fantasy or Modren it is obvious which ones to pick out. It get more complicated with Sci-Fi but the genre books like GURPS Space and GURPS Traveller give you template that help greatly.
Template are a great innovation in late 3rd Edition and are implemented Fully in 4th edition. They don't give you any benefits but organize skills, advantages, disadvantages, perks and quirks in a coherent way. For example in GURPS Traveller the Starship Engineer Template give you clear choices to make up that type of characters.
Campaigns with detailed background can use them to allow players to make characters that are part of specific Organizations. You can see an example here. It is a template I use for making characters that are Myrmidons of Set. It predates the one in the Majestic Wilderlands supplement by several years so it differs a little.