There were daily Liza updates and photos in the News and Post back in 1975. (PS: the Post was a semi-reputable paper back then.) At Liza's request, Gwen's name remained in the ads and on the marquee, but there was a lobby sign and a pre-show announcement.
Lenora Nemetz was Gwen's understudy and played the role for maybe 10 days. This was a break for her and she went on to some other shows and star status in her hometown of Pittsburgh.
Chicago had gotten off to a shakey start. It got decidedly mixed opening night reviews and word-of-mouth was not great, plus the theatre party ladies felt the show was a bit raw for their groups so they weren't buying. And the original was a lot more sordid and darker than the revival.
There was also the negative publicity because Chicago was the first show to break the $15 orchestra price barrier which was a level that had held for maybe 4-5 years. $17.50 and even $15 was big money to people (especially people like you and me) in those days.
And yet another negative, A Chorus Line had opened 2 weeks prior to major acclaim and super hit status. It had a huge and growing advance sale and was getting a lot of buyers who might have bought Chicago tickets. Chicago had become the homely step-child.
With all the publicity that Liza generated, both in NY and nationally, her appearance is thought to have "saved" the show from an earlier death. She paid off a lot of debts to Kander, Ebb and Fosse when she did Chicago.
The party line on Gwen has always been that she swallowed something--a feather or a piece of the glitter that was used at the end of the show. There were also rumors, never proved, that the problem was a node on her vocal chord that needed to be scraped off and then given time to heal. We'll probably never know.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher