In order to compute the coverage in terms of unittest of your catkin workspace, please do the following:

catkin_make -DCMAKE_CXX_FLAGS=--coverage -DCMAKE_SHARED_LINKER_FLAGS=--coverage -DCMAKE_EXE_LINKER_FLAGS=--coverage -DCMAKE_BUILD_TYPE=Debug .. && catkin_make run_tests && gcovr -r .