Remember, when communicating with a computer, everything eventually boils down to ones and zeroes, including the letters of the alphabet. As far as the computer is concerned, every letter, every number, and every piece of punctuation is just a number. A character set is just a system of correspondences between numbers and the characters they represent. For instance, in many character sets, capital A is represented by 65 (binary 01000001, hex 41). This is true for the famous character set ASCII, and it's also true for the character set ISO 8859 Latin-1, which Glk uses.
Characters 32 through 126 of ISO 8859 Latin-1 are guaranteed to work on any platform. (Consult the ISO 8859 Latin-1 chart to see what the various numbers stand for.) Some platforms will support a number of other characters. Some won't. Determining which is which is what the gestalt_LineInput, gestalt_CharInput and gestalt_CharOutput constants are for.