“Today’s gossip is tomorrow’s headline” is what gossip columnist Liz Smith frequently credited (since the 1970s) to earlier gossip columnist Walter Winchell (1887-1972).  Winchell provided the original story for the film Broadway Through a Keyhole (1933), and advertising copy for the movie began:
 
“Walter Winchell, Broadway columnist whose today’s gossip is tomorrow’s scandal, is author of the story of ‘Broadway Through a Keyhole,’ ...”
 
“Today’s gossip is tomorrow’s scandal” is close, but it’s not known if Winchell ever wrote “today’s gossip is tomorrow’s headline” that he is credited for today.
 
     
Wikipedia: Walter Winchell
Walter Winchell (April 7, 1897 – February 20, 1972) was an American newspaper and radio gossip commentator.
 
Wikipedia: Broadway Through a Keyhole
Broadway Through a Keyhole (1933), also billed as Broadway Thru a Keyhole, is an American musical film, perhaps most notable for the New York City speakeasy proprietress Texas Guinan appearing as a fictionalized version of herself, and an early appearance by Lucille Ball. The film is based on an original story by Broadway columnist Walter Winchell.
 
3 December 1933, The Sunday News and Tribune (Jefferson City, MO), “Broadway Seen Thru a Keyhole at Miller,” pg. 11A, col. 5:
Walter Winchell, Broadway columnist whose today’s gossip is tomorrow’s scandal, is author of the story of “Broadway Thru a Keyhole,” which opens Thursday at the Miller Theatre for a three-day run..
 
18 December 1933, Corsicana (TX) Daily Sun, “Stars at Palace Theatre Tuesday,” pg. 12, col. 2:
Walter Winchell, Broadway columnist whose today’s gossip is tomorrow’s scandal, is author of the story of “Broadway Through a Keyhole,” produced by the 20th Century and featuring Constance Cummings, Russ Columbo, Paul Kelly, Blossom Seeley, Abe Lyman and his band, Gregory Ratoff, Texas Guinan, Eddie Foy, Jr. and Frances Williams at the palace Tuesday.
   
23 January 1934, The Evening Tribune (San Diego, CA),  “Comedy Stars in Cabrillo Picture,” pg. 8, col. 3:
Walter Winchell, Broadway columnist whose today’s gossip is tomorrow’s scandal, is author of the story of “Broadway Through a Keyhole,” produced by the 20th Century and featuring Constance Cummings, Russ Columbo, Paul Kelly, Blossom Seeley, Abe Lyman and his band, Gregory Ratoff, Texas Guinan, Eddie Foy, Jr. and Frances Williams at the Cabrillo theatre today and Wednesday.
 
Google News Archive
8 August 1976, The Ledger (Lakeland, FL), pg. 2D, col. 6:
For as Walter Winchell, once the dean of gossip columnists, once observed, today’s gossip is tomorrow’s headlines.
 
Google Books
What They Said in 1978:
The Yearbook of Spoken Opinion
Beverly Hills, CA: Monitor Book Co.
1979
Pg. 336:
Liz Smith
Newspaper columnist
What is gossip but unsubstantiated rumor? People are universally interested in gossip. I think gossip is just news running ahead of itself in red satin dress. As much as I detested Walter Winchell, there was some truth in his saying, “Today’s gossip is tomorrow’s headline.” We certainly have seen that proved a lot—although I don’t believe where there is smoke there is necessarily fire.
Interview, Dallas/
The Dallas Times Herald, 8-3:(E)10.
 
7 April 1995, The Capital Times (Madison, WI), pg. D1, col. 1:
TODAY’S QUOTE “Today’s gossip is tomorrow’s headline.”—Walter Winchell
 
Google Books
And I Quote, Revised Edition:
The Definitive Collection of Quotes, Sayings, and Jokes for the Contemporary Speechmaker
By Ashton Applewhite, William R. Evans and Andrew Frothingham
New York, NY; Thomas Dunne
2003
Pg. 352:
Today’s gossip is tomorrow’s headline. — Walter Winchell