Gregorian Chant is written on a four-line stave. The notes, called neumes, are shown as squares or diamonds.
At the left hand side of the stave will be found either a symbol that looks like a telephone handset or a C, which signifies Doh, or else a symbol that looks something like the letter E with the middle stroke pointing to the left, which signifies Fa. But Doh and Fa can be any pitch that happens to be convenient for the singers, depending on the range of the music. This is very handy. If people find they cannot sing the high or low notes, the music can always be sung at a different pitch to suit their voices.
This means that there are no keys in the modern sense of the term. There are, instead, eight modes, each with a characteristic mood or mode, based on a sequence of full tone and half-tone intervals. There is a set of psalm tones for each mode. Mode 5 corresponds to the major keys in modern music and has a bright sound, whereas mode 4 is reflective and sombre.
There is no necessary correspondence between the notes as written on the four line stave and the notes on a five line stave, where each note has a defined pitch.
The notes and groups of notes are known as neumes. It is easier to sing chant from these neumes than from a transcription into modern notation. This is because some of the information about how to sing the music is lost in the transcription, as modern notation cannot describe what is meant. It is therefore important to learn how to sing from the correct notation. Once learned, it is easier, not least because there are only four lines on a stave instead of five.
For more information on how to read Gregorian chant see here