If you sight-read/sing modern notation already, Gregorian notation can be easy to learn. It’s works on a solfeg system, so you just need to know your intervals and sing the chant on the starting pitch that you’re comfortable with.
There are two clefs used. The one that looks like a backwards C is a Do-clef, and it’s position tells you where Do is. Same principle with the other clef, the Fa-clef, for Fa.
You treat all the squares and diamonds like eight notes, but if they have certain markings (explained in the link below), you would hold them just a bit longer. If you see a squiggly line, the square note that precedes it is held, while the squiggly is just treated like a normal note.
The modern notation equivalent for the chant in the following link should only be a guide to the rhythm and intervals, and as I mentioned before, you can sing on any key you want that’s in your range.
Gregorian Notation Guide