Your very link states that it's okay to distribute combinations of GPL-licensed code with code licensed under another, GPL-compatible license (which MIT is).
Suppose that you wrote an MIT-licensed Hello World function (one work) and have a copy of someone else's GPL-licensed FizzBuzz function (a separate work). If you distribute both works together (in the same repository, for example) then the combination (a new work) must be under the same GPL version (or a later version, if the license notice for the FizzBuzz function that function says something like "or any later version" [1].
If you write and then distribute a Fibonacci function which calls someone else's GPL'd add function (or your Fibonacci function body includes code from that GPL'd add function), then your distributed Fibonacci function must be under the same GPL version (or a later one, if the license grant lets you).
Comments
The same answer in the FAQ (https://www.gnu.org/licenses/gpl-faq.html#WhatDoesCompatMean) contains a caveat mentioned by Tomte's sibling comment (https://news.ycombinator.com/item?id=39840391): the distribution as a whole must be GPL-licensed.
Suppose that you wrote an MIT-licensed Hello World function (one work) and have a copy of someone else's GPL-licensed FizzBuzz function (a separate work). If you distribute both works together (in the same repository, for example) then the combination (a new work) must be under the same GPL version (or a later version, if the license notice for the FizzBuzz function that function says something like "or any later version" [1].
If you write and then distribute a Fibonacci function which calls someone else's GPL'd add function (or your Fibonacci function body includes code from that GPL'd add function), then your distributed Fibonacci function must be under the same GPL version (or a later one, if the license grant lets you).
[1] https://www.gnu.org/licenses/gpl-3.0.en.html