Maybe it was originally Billy Gasconade? Means braggart. Billy 'Liar'.
There is a local story that the Jesse James song was written here in Franklijn county, Missouri. We here in Franklin county once claimed the Gasconade river as ours until the state legislature made Franklin county smaller by slicing off our western border, calling the truncated result Gasconade county. Franklin county was the largest county in Missouri until then. Franklin was also a 'seccessh' stronghold except for the town of Hermann whose German population was staunchly pro-union "I go to fight mit Segal" Hermann became Gasconade county's county seat. Owensville, Gasconade, Rosebud and Hermann are the principal towns in what is one of Missouri's poorest counties but also a center for Missouri wineries with some excellent, award winning wines available.
Oddly enough, hardly anyone here sings the venerable 'Jesse James' anymore, though 'black flaggers' and 'bushwackers' would find support and sympathy even today. Some here STILL don't like Kansas.