The story
From Glock + -izzy (suffix forming hip-hop-sounding words). The "hot dog" sense comes from comparison to the extended magazine of a pistol. Both of these senses were ascribed to the Washington metropolitan area until piquing the curiosity of the world in June–July 2020 when internet users endeavoured to upload pictures containing sausage buns just to say the term. The "penis" sense was a natural progression from "hot dog".
Etymology from Wiktionary, CC BY-SA 4.0.