Gtest is the Google framework for running C++ unit tests. It is a pure C++ framework. No ROS environment is available. See: Configuring rostest if your tests need a running roscore.
Declare each gtest like this:
if (CATKIN_ENABLE_TESTING)
catkin_add_gtest(test_your_node tests/test_your_node.cpp)
target_link_libraries(test_your_node ${catkin_LIBRARIES})
endif()
This example assumes your tests are defined in the tests/ subdirectory in your source tree.
If other libraries are needed to compile your test program, see Building and installing C++ executables for details.