I managed to run Gnuk on GD32F103, with some fixes. It takes 6 seconds for RSA4096. For 100 invocations of gpg --decrypt with cv25519, it takes 7.5 second (for STM32F103 @ 72MHz, it takes 21 second). For 100 invocations of gpg --detach-sign with ed25519, it takes 4.4 second (for STM32F103 @72MHz, it takes 10 second).