What are some alternatives?
When comparing AudioKit Synth One and KORG Gadget, you can also consider the following products
Sampulator - Make (and record) beats on your keyboard
DRC - Polyphonic Synthesizer - Cross-platform, virtual synthesizer for iOS that recreates the sound of classic, analog synths.
Ableton - Music production software and hardware.
Caustic - Caustic is a music creation tool inspired by rack-mount synthesizers / samplers rigs.
Splice Beat Maker - Make and share beats in your browser
Poison-202 - Vintage MIDI synthesizer for Mac and iOS