The release dates & the years that movies win Oscars always seem kind of screwy. This might be the reason pop culture and some of us (including me & my poor befuddled brain) get the years mangled. "Snow White" is a special case, though.
1937 - "Snow White" had its Los Angeles premiere on December 21st, 1937. It went into nationwide release on February 4,
1938.
1938 - "Snow White" nominated for Best Music/Score for the 1938 Academy Awards.
1939 - Walt Disney was awarded a set of Honorary Academy Awards at the 1939 ceremony consisting of one full-sized "Oscar" and 7 miniature "Oscar" statuettes.
1939 - "Gone with the Wind" premiered in December of 1939 (dates ranged from the 15th in Atlanta to the 28th in Los Angeles). In 1939/1940 the film only had limited advance-ticket engagements & seems to only have finally gone into general-release sometime later, perhaps in 1941.
1940 - "Gone with the Wind" nominated for (and won) numerous Oscars at the 1940 Academy Awards, beating out many other worthy contenders for the honors, including the classic "Wizard of Oz".
I did have to refresh my memory with many searches, including IMDb.Com, the (sometimes unreliable) Wikipedia, fansites, etc. Actually *proving* the "GwtW" staggered release dates (limited, genereal, etc) from good sources is not an easy task.
Yeah, I do like to do research when I'm not DIS'ing

...
agnes!
PS - Here's an additional piece of Oscars trivia. Viktor Fleming directed "Wizard of Oz"(1939) and was also the last of four directors who worked on "GwtW".