[YetAnotherNick]

MIT code could be used in GPL, but other way round is not possible. As long as there is a line of code with GPL license, entire project will be GPL.

[p_l]

Not really, only when it can be shown that the project is derivative of the GPL code (for example, hard dependence on GNU-only library for some of its features vs. "one of the multiple possible implementations we can use is GPLed")