I had this issue also. For me, the problem is that GAppM-10.24.0.tgz has files inside it with very long names. These long names cannot extract to the C:\Users{username}\AppData\Local\XamarinBuildDownloadCache\ directory because it violates the MAX PATH length of 260 characters. (At least on Windows 10). So it hangs trying to create a file it cannot create.