FindGTest.cmake give up before finding GMock if it finds a config file
If the GTestConfig.cmake file from upstream is found, it is accepted and the module exits. https://gitlab.kitware.com/cmake/cmake/-/blob/v3.23.1/Modules/FindGTest.cmake#L193-207 However, at least on Debian Bullseye, the gmock targets are not included in that, and the module doesn't check to see if gmock exists before exiting.
On bullseye, there are no gmock targets files in gmock-dev: https://packages.debian.org/bullseye/amd64/libgmock-dev/filelist but there is in libgtest-dev: https://packages.debian.org/bullseye/amd64/libgtest-dev/filelist (and it doesn't mention gmock at all)
In testing/bookworm, there are some cmake target files, optionally included by the main gtest one. https://packages.debian.org/bookworm/amd64/libgmock-dev/filelist so there, I think the problem wouldn't be observed.