Some cancer forms have their own grading schemes, but there are 3 grades in general:

Grade 1 cancer cells look very similar and grow gradually (low grade) to normal cells.

Grade 2 the cells do not look like regular cells, but grow faster than average (intermediate grade)

Grade 3 the cancer cells appear very abnormal and develop rapidly (high grade)

Some programs have grades higher than 3.

GX means doctors are unable to determine the grade. Often known as an undetermined grade.