How was the nuclear device detonated at Trinity nicknamed?

Little Boy and Fat Man were the names for the two nuclear bombs used operationally, on Hiroshima and Nagasaki. But there was an earlier bomb, developed for the nuclear test, code-named Trinity. This test was conducted by the United States Army on July 16, 1945, as part of the Manhattan Project, on what was then the USAAF Alamogordo Bombing and Gunnery Range (now part of White Sands Missile Range).

The term "Gadget" was a laboratory euphemism for a bomb, from which the laboratory's weapon physics division, "G Division", took its name in August 1944. At that time it did not refer specifically to the Trinity Test device as it had yet to be developed, but once it was, it became the laboratory code name. The Trinity Gadget was officially a Y-1561 device, as was the Fat Man used a few weeks later in the bombing of Nagasaki. The two were very similar, with only minor differences, the most obvious being the absence of fusing and the external ballistic casing. The bombs were still under development, and small changes continued to be made to the Fat Man design.

The picture is of the bomb known as "Gadget"