why is it called tgs on 30 rock
It was originally called "The Girlie Show".
30 Rock
Liz Lemon (Fey) is head writer and showrunner of the NBC sketch comedy series "TGS with Tracy Jordan" (originally called "The Girlie Show"), produced in Studio 6H in 30 Rockefeller Plaza. 
wiki:16051277
30 Rock [SEP] Premise
Liz Lemon (Fey) is head writer and showrunner of the NBC sketch comedy series "TGS with Tracy Jordan" (originally called "The Girlie Show"), produced in Studio 6H in 30 Rockefeller Plaza. She supervises cast and crew including star Jenna Maroney (Krakowski), her best friend, while working with network executive Jack Donaghy (Baldwin) and page Kenneth Parcell (McBrayer). In the first episode, Donaghy forces Lemon to hire the unpredictable Tracy Jordan (Morgan) as costar. Episodes often depict conservative Donaghy and liberal Lemon's disagreements, Maroney's attempts to become a film star, Jordan's eccentric behavior, and the characters' romantic and personal lives. The show satirizes the cultures of NBC, parent company General Electric, and the entertainment industry.