[colejohnson66]

I’m not even sure a grant is necessary. As the author, you can relicense your work whenever you want. That includes relicensing for a certain platform. So, release your GPLv3 source code, but release the “proprietary” version on the App Store.

[greggman2]

If you're the sole author yes. If you took any contributions then no unless you got all the contributors to accept your proprietary license for their contribution.