I very well could have misread. DisneyWiki had this to say on Gustav:
Gustav the Giant is the main villain of the Mickey Mouse short Brave Little Tailor. The Giant constantly terrorized the kingdom, unopposed. When Mickey Mouse, the Brave Little Tailor was ordered to slay the Giant by the King (Minnie Mouse's father), who thought he killed seven giants in one blow (flies, actually), Mickey used his tailoring skills to defeat the giant. The giant was last seen sleeping, his breath powering the Merry-Go-Round, which Mickey and Minnie were riding.
I have never heard of Gustav either, which is why I say I may have misread it back then.