Classification
Along with cell shape, Gram staining is a rapid diagnostic tool and used to be used to group species at the subdivision of Bacteria.
In fact, historically the kingdom Monera was divided into four divisions based on Gram staining: Firmacutes (+), Gracillicutes (-), Mollicutes (0) and Mendocutes (var.). Since 1987, the monophyly of the Gram-negative bacteria has been disproven with molecular studies. However some authors, such as Cavalier-Smith still treat them as a monophyletic clade and refer to the group as subkingdom "Negibacteria".
Read more about this topic: Gram-negative Bacteria