Dating pianos purely on the basis of serial numbers is often not as simple as websites make it out to be, but you may be able to find numbers imprinted on removable wooden parts such as the music desk or legs.
The firm was originally just George Guild until 1861, when he started his partnership with Briggs. In 1867, George M. Guild went into partnership with Mr. Church, a partnership which ended in 1890, when Church withdrew. The name of the firm was changed to “The Guild Piano Company”.
Their 1891 ad for The Guild Piano says they had been made in Boston for nearly 30 years. Washington Mass. is 129 miles from Boston Mass., so it is not clear whether they were ever made in Washington.
They continued to build pianos well into the first quarter of the 20th Century, including grands and uprights, as no so-called "square grands" are known to have been made by anyone after 1900.
Nobody can tell you anything about an individual piano just from a name, but sadly, this is one of many redundant makers that have left very little information behind. If you can post photos here to show what the WHOLE piano looks like, we may be able to say more.