the only reasonable difference is that gtest_main.lib provides a default implementation of a test application entry point (i.e. main function):
Citation from Getting started with Google C++ Testing Framework:
“[…] maybe you think that writing
all those main() functions is too much
work? We agree with you completely and
that’s why Google Test provides a
basic implementation of main(). If it
fits your needs, then just link your
test with gtest_main library and you
are good to go.”
If you want to write your main function yourself – you should link with gtest.lib.