The fictional sketch show within 30 Rock is TGS. Its full name is TGS with Tracy Jordan, and the initials are the name most commonly used by the characters and viewers.
The program is the fictional live sketch-comedy series produced at NBC's 30 Rockefeller Plaza. Liz Lemon serves as its head writer, while Tracy Jordan and Jenna Maroney are its volatile stars. The show's behind-the-scenes chaos forms the central workplace setting of 30 Rock.
TGS began as The Girlie Show, the program created by Liz and Jenna before Jack Donaghy brought Tracy Jordan into the cast. That change led to the renamed title, TGS with Tracy Jordan. The Girlie Show is therefore an earlier version of the program, not the final name usually sought by this question.
The fictional series deliberately echoes Saturday Night Live, which was also produced at 30 Rock and helped inspire Tina Fey's sitcom. TGS is not a real NBC program; it exists as the show-within-a-show around which the comedy's storylines revolve.