It seems Clark Gable (1901-1960) was a friendly autograph signer during his life as authentic autograph are not as rare but be careful on secretarial autographs on the market.
Clark Gable used two slightly different styles to sign official documents and fan autographs.
Signature for official documents like contracts and checks. They are ALLWAYS signed with an open "G"
Signature for fan signed photos given In Person or by mail. They are ALLWAYS written with a large round "G" loop.
The following signatures are secretarial signed.
Scan 1 shows a secretarial signature from ca. late 1930's or 1940's used in the USA on 8x10inch photographs
Scan 2, 3, 4 and 5 shows secretarial autographs sent out by the "Filmstar-Sekretariat Hans Dangers, Berlin-West" in the late 1950's and early 1960's.
They are normally only used on 3,5x5,5inch or 4x6inch postcards. The secretary "autographs" must be detected in the prename as Gable is closely done to authentic examples. Clark Gable NEVER underlined his name in full from the C and the "a" in Clark is more written like two small connected "e" in authentic examples.