Garbo and Clark were in a 1931 movie "Her Rise and Fall" and, apparently, developed a tremendous dislike for each other. So, the fact that in this scene Garbo dominates Gable keyed into part of the joke at the time.
The 1935 film "Anna Karenina" was Garbo's showcase film. However, the aspects of her acting that were appealing became a liability as the movie industry evolved. Her "I want to be alone" line became the mission statement for her later life. Swedish by birth, she was criticized for not taking a public stance against Hitler -- which tarnished her public image and led to fewer roles being offered her. It was learned later that she was secretly involved in gathering information for the U.S. Government, but by the time the information became public it was too late to counter the PR damage done years before. [By the way, if you want to read a really interesting story, check out actress and scientist Hedy Lamarr and her role as co-inventor of spectrum spread telecommunications.]
Clark Gable was starting to develop the rugged "bad boy" womanizer image before "Autograph Hound" (1939) and "Gone With The Wind" also released in 1939. "It Happened One Night" (1934) earned Gable an Oscar and "Call Of The Wild" (1935) both put Gable to the front of the pack when it came to fulfilling the everyday role of male Hollywood star... So, having Gable dominated by Garbo was part of the joke in "Autograph Hound."
While we recognize Gable much more quickly than Garbo, it's Garbo that was the more significant actor. Her portrayals are more widely cited at key examples of acting -- as far as I can tell.....
Greta Garbo & Clark Gable Cel on Master Background
Greta Garbo & Clark Gable Pencil
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From Autograph Hound (1939). Greta Garbo has Clark Gable in a dip. Both are looking at each other. I believe that this is where they hear that Donald Duck is on the set. Directed by Jack King. SeqID-1206. See SeqID 0025 for a near identical pencil sketch
From "Autograph Hound" (1939). In this pencil sketch, Garbo has Gable in a dip. Both are looking off the screen at sounds being made by Donald Duck. A Donald Duck film directed by Jack King. [Image: 8.5"W x 7.5"H; Frame: 16.5"W x 15.5"H] SeqID-0025 8/3/2005