The question becomes really simple if you are releasing as open source and everything using your code is also open source: everything will be fine. If your code is proprietary or is used in proprietary software, it becomes a bit more complicated. The main question is whether or not you or others are using your MIT licensed code in a proprietary manner.

You can link to code covered by the GPL even in proprietary systems as long as the license is compatible. The MIT license is generally compatible with the GPL, but I recommend asking FSF directly as these multi-license situations can be tricky.

The question becomes really simple if you are releasing as free and open source and everything using your code is also free and open source: everything will be fine as long as your code is using a compatible license. If your code is proprietary or is used in proprietary software, it becomes a bit more complicated. The main question is whether or not you or others are using your MIT licensed code in a proprietary manner.

You can link to code covered by the GPL even in proprietary systems as long as the license is compatible. The MIT license is generally compatible with the GPL, but I recommend asking FSF directly as these multi-license situations can be tricky.