(This measure has not been amended since it was introduced. The summary has been expanded because action occurred on the measure.) Gains in Global Nuclear Detection Architecture Act (Sec. 2) This bill amends the Homeland Security Act of 2002 to require the Domestic Nuclear Detection Office, in conducting research and development to generate and improve technologies to detect and prevent the illicit entry, transport, assembly, or potential use within the United States of a nuclear explosive device or fissile or radiological material, to: develop and maintain documentation that provides information on how the Office's research investments address gaps in the enhanced global nuclear detection architecture and with research challenges identified by the Director, and that defines in detail how the Office will address such research challenges; document the rational for prioritizing and selecting research topics; and develop a systematic approach for evaluating how the outcomes of the Office's individual research projects collectively contribute to addressing its research challenges.