As far as I know, the GPL is for licensing code in source and binary formats, not the formats of data files manipulated by said code.
An instructive example is GCC. GCC is licensed under the GPL, but the source code that goes into it and the binaries that come out of it are not affected by that license (its version of the GPL even has a special exception to allow linking against its runtime library). So normally its license only comes into play if you want to change GCC or make a program using the code in GCC.
There is such a thing as a "patent-encumbered" file format. This most famously applies to audio and video file formats like MP3 and MP4 where the inventor(s?) spent a heck of a lot of R&D money figuring out the compression algorithms used by those formats. But in these cases the use of the format doesn't force you to change your license, it forces you to give the patent holders money (if you exceed certain usage thresholds).
All-in-all, I would be very surprised if a file format for a plugin for free software had any "infectious" license/patent/whatever. But there may be political/social/cultural reasons to use GPL (for all we know the author of the plugin may have strong feelings about free-as-in-speech software and copyleft licensing). You're probably fine using MIT, but IANAL, and it can't hurt to ask the orgmode author directly.