Long famous for its range of planers and plano-milling machines, G.A.Gray Jr. & Co. was founded in 1881 by George A.Gray Jr. in Cincinnati, Ohio, and reorganized as the G. A. Gray Co. in 1886.
Of massive construction and great rigidity, the 'plano milling machine' evolved from the early, simple - but oh-so-useful - small hand-operated planers such as these Victorian examples from the 1800s by Brittain,  Fomm, and Plant. Simultaneously, larger machines with power feeds were also in production, a good example of the variety available being those made by the English Company Milnes.
It did not take long before it was realised the overhead rail could hold not just one,, but two or even three toolholders - and the vertical supports used to house them as well. Hence, a job -  or multiple sets of identical jobs - could be set up on the table and several cuts taken simultaneously. From there, it was only a small step to replace the cutting-tool holders with powered spindles that held rotating cutters or grinding wheels - hence the terms "plano-miller" and "plano-grinder".