Gill first became noted for his landscape illustrations and watercolors, featuring subjects such as Native American pueblos in addition to his main focus on Washington, D.C.

Characterized as detailed and meticulous in his landscapes, Gill captured views of working-class and rural areas of Washington not commonly depicted in art of the period.

In this role, he produced thousands of photographs of Native American delegations for the Bureau, including notable figures such as Geronimo and Chief Joseph.

While Gill's costuming of delegates was considered salvage ethnography in the period, it has been criticized in modern studies for reinforcing contemporary stereotypes and misrepresenting his subjects and their cultures.

He briefly worked as a typesetter before finding employment as a draftsman for the Office of the Supervising Architect for the U.S. Treasury, specializing in ornamental ironwork and tiles.

[6] United States Geological Survey (USGS) chief of illustration William Henry Holmes hired Gill as a paleontological draftsman in 1884.

Holmes, a fellow watercolor painter, came to greatly respect Gill's artistic work, later claiming that "as an illustrator in pen, pencil, and water-colour, and as a photographer, he had few equals."

[7] During his initial time with the Bureau in the late 1880s, Gill produced paintings of southwestern pueblos, departing from his prior focus on Washington.

[9] Gill's watercolor work was favorably received by contemporary art critics, with a reviewer for the Brooklyn Daily Eagle describing his landscapes as "airy, clean, silvery" and distinct from other watercolorists.

[11] In 1983, art historians Andrew Cosentino and Henry Glassie wrote favorably of the extreme clarity and attention to detail shown in Gill's paintings.

[6] Cultural historian Lisa Goff praised his illustrations, writing in 2016 that his sketches of Washington, D.C. were "not nostalgic or sentimental, but [...] studies of a vanishing landscape", made without demeaning the working-class shanties.

[16] In this position, Gill's work consisted of portrait photography of Native American subjects, primarily delegates to the capital, at a rate of hundreds of individuals per year.

[17] He also extensively classified and cataloged the resulting photographic negatives along ethnolinguistic lines, at one point re-cataloging the entire BAE archive of Native portraits.

[21] Joseph Henry, the first secretary of the Smithsonian Institution, ordered that Native delegations should be photographed without headdresses so that the "configuration of the head" could be recorded.

[22] William Henry Holmes, succeeding Powell as Chief of the BAE in 1902, also emphasized the "anthropometric elements" of the photographs, placing items with known sizes around subjects in order to allow for facial and cranial measurements to be taken.

However, as ethnographic and historical purposes were increasingly pursued, subjects were often depicted with headdresses, wearing Native dress and holding tools or artifacts associated with their tribe.

The following year, he partnered with William John McGee on an expedition to Arizona and New Mexico, photographing members of the Akimel O'odham, Cocopah, Seri, and Tohono Oʼodham.

He continued work as illustration editor for various bureaus within the Smithsonian in addition to the BAE, including the United States National Museum.

[14] Gill's early portraits, especially full-body and group photographs, were characterized by Glenn as inartistic, featuring stiff poses in front of "wrinkled and ill-draped backdrops".

[30] Gill worked within the ethnographic, historical, and anthropometric demands of the BAE alongside his own artistic goals, with full body shots mainly serving to document clothing rather than the subject themselves.

[30] However, Glenn saw a significant improvement over the course of Gill's early career, with his close portraits showing greater artistic intent than full-body shots.

He would take great care to edit pieces for prints and publication: a portrait of Wolf Robe was heavily cropped in order to create a more artistic design.

The provision of props and clothing for the delegate's portraits, alongside deliberate posing, were accepted during the period as a means to record what was perceived as a vanishing culture.

[34] In other cases, Gill photographed "show Indians", performers who wore clothing intended to appeal to public conceptions of Native dress.