When you link GPL code with your proprietary code into a binary (shared address space), and distribute the binary to people, those people are entitled to your proprietary source code.
When I compile my GPL code with a copy of somebody else's MIT code into a binary (shared address space), other people's copy of that MIT code is still MIT licensed.
When I compile my GPL code with a copy of somebody else's MIT code into a binary (shared address space), other people's copy of that MIT code is still MIT licensed.