To be pedantic, if you're mixing your code with GPL code and distributing the result then you're obligated to make it all GPL in order to follow the license, but it does not by itself become GPL.
More precisely you can make it any license that is GPL-compatible (including MIT), and distribute the source in order to follow the license. But it's okay for people to strip the GPL bits and only follow the permissive license on the rest.
This has been done for QEMU in the past, for example (QEMU for historical reasons is a mishmash of GPLv2-compatible licenses, which is fine as long as the combination is distributed under the rules of GPLv2).
