Upvote:4
There's not much that can be said that hasn't already.
That's the uniform of a enlisted United States Army "doughboy". The uniform dates from between the first and second world wars.
The insignia is that of the US Army Band.
Note: The above insignia is "band leader". I assume the star above is the reserved for band leader.
That's as much as I can get without more details. I recommend you make a request to the US National Archives, you should be able to get details about your great grandfather.