For the simplest possible usage example, just include gtest/gtest.h and add this to your application entry point:
int ret = RUN_ALL_TESTS();
Now, each test you wanna add is declared like so:
// test goes here
The framework provides a bunch of macros for value tests and assertions, a very simple example being:
ASSERT_EQ( 1, someValue );
where the test fails and spits out an instructive error message in case someValue != 1.
I've found this framework to be extremely simple to learn, but once you scratch the surface you'll find it also has quite a few advanced features such as listeners, reflection, type-parametrized testing etc. to keep you busy for quite some time.