For my regular collection books, if I do know the Guided reading level, I enter it in the 521 filed. 521a is the GR letter and 521b is the term " Guided Reading Level." Hopefully, it is already in a purchased MARC record.  As far as our shared library (GR textbooks/trade books), we purchase the F and P book to level the library.  Of course, not all are there. Many of the companies have started to list the GRL online too.  I search where I can.  However, adding GRL to the regular MARC records is not a priority.  I do it when I have time!

--Deb