Note: Since Ive written this text it was brought to my attention that MA Lloyd has a similar, albeit much longer essay on literacy in GURPS, which can be seen at http://www.io.com/~ftp/GURPSnet/AdsDisSkills/Languages/languages.literacy. This is an excellent work, very detailed and perhaps somewhat too detailed for some campaigns, but it also includes simpler, more playable variations of the rules it presents. Therefore, although Ive decided to leave my rules here (in case someone likes them :), I strongly recommend Mr. Lloyds text. He have also written a similar essay on language skills, as well as a comprehensive list of worlds languages and language families -- I think you should find them in the same directory.Some time ago there was a debate on rec.games.frp.gurps about whether should a character have a separate Literacy advantage for each language he knows, or one Literacy covers them all. Well, I gave that issue some thought, and here is my two cents worth on that subject.
When we look at all the writing systems which were invented by human race during its history, we can see that all of them were variants of only four of general categories of alphabets:
With all their differences, various alphabets within each of the above groups share the same principle, and that is what counts. A person who is familiar with Latin alphabet will quite easily get a grasp on, say Cyrillics, while Katakana or Egyptian hieroglyphs might give him a bit of a headache. So one with a Literacy advantage is automatically proficient with writing systems of all languages he had learned at least a little (i.e. where he has at least 1/2 pts; defaults dont count, unless they have the same or very similar alphabet as the language they default to), as long as that system falls into the same category as the system used by his native language.
To be able to read a different kind of alphabet, one must take another, separate Literacy advantage, at the same cost. It effectively means that there are four separate Literacy advantages -- one for each category -- although it is possible to add more in campaigns that require them (for instance, I have once envisioned a culture which uses no writing, but they record their thoughts in form of abstract artistic paintings -- arabesques -- which can be read like any other writing, but precise interpretation depends on the reader as much as on the writer). It is also possible to have only one Literacy, if all the world uses the same category (like in Tolkiens Middle-earth) or maybe two if you have only two dominant cultures (in a Space campaign, for instance).
Actually, a skill with a language is not required for someone to learn an alphabet. Most alphabets can be used with other languages that they were designed for, and various languages can be transcribed using various categories of alphabets; a good example is Japanese, which equally uses Chinese pictograms (for most writing needs), Katakana and Hiragana (for transcribing foreign names and other occasions when pictograms dont suffice, and Latin (when transcribing Japanese names and other words for Europeans).
In former Yugoslavia, for instance, there were two languages -- Croatian and Serbian -- which are similar enough that speakers can easily understand each other. However, they use different scripts: Croatian uses Latin, while Serbian uses a form of Cyrillics. During communist reign the schools taught the children both of these scripts, so today there is many Croats (I am an example) who can easily read Cyrillic books published in Serbia -- and even the Russian ones, although the language itself might not be understood. Another example is the Tolkiens transcriptions on title pages of all his books, where he used Elven letters to write a text which is actually in English.
If a character wants to learn a writing system without learning its language (or when there is no language to begin with: Etruscan script, for instance, is understood, while the language is still a mystery; there are also several code-systems which are actually a different alphabet -- Scottish pigpen is the best example), it is possible as long as the system falls into the same category as the Literacy advantage that the character already have. If the desired system is in another category, the character first must take the appropriate Literacy (or Semi-literacy) advantage.
To learn an alphabet, the characters needs to spend no points -- its value is already covered with the Literacy advantage. It still takes time to learn the alphabet, however: the character must take 200 hours learning it (GM may alter this period if he feels that an alphabet is more or less complicated; for instance, for a European or American character, Chinese pictograms would be a lot harder to learn than, say, Cyrillics or pigpen).