What happend to /usr/bin/gmcs?
gmcs is deprecated and was removed from Mono 4.0.
Also Starting with Mono version 2.11 a new unified compiler mcs is available. It replaces all previous runtime specific compilers (gmcs, dmcs, smcs). They still exist (as scripts only) to ease the migration path to mcs but we strongly recommend to use mcs. please don't use gmcs
we need to focus on bug 580316 then