/usr/bin/gmcs is in mono-mcs 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2
#!/bin/sh exec /usr/bin/mono $MONO_OPTIONS /usr/lib/mono/4.5/mcs.exe -sdk:2 "$@"