Holst Two come to mind, Gustav Mahler, Austrian, 1860-1911 and Gustav Holst,English "yes!" 1874-1934.
Gustav
There are two Holst or Malher.
Gustav Holst
He was born and lived in England, despite his name [originally Gustav von Holst]. He was a popular composer of classical music.
Gustav Theodore Holst (21 September 1874 - 25 May 1934) was an English composer and was a music teacher for nearly 20 years.
I would suggest the middle name "Hans" for Gustav. It has a strong, classic sound that complements the traditional feel of Gustav.
Do you mean Jupiter, an op in "The Planets", composed by Gustav Holtz?
Gustav Holst
Gustav Holst.
Perhaps you mean Gustav Klimt.
The composer's full name is Michel Legrand.
Gustav Mahler