I must say, I am no expert in Kotlin but the source code looks really clean and tidy. I've tried the plugin and, honestly, it seems like a great replacement for PEX, at least the somewhat basic way I used the plugin.
One suggestion would be to support HEX gradients and what not. To add that to my plugins I checked this repo out: https://github.com/funnyboy-roks/Gradient-Generator-Plugin
There's multiple classes that do the same thing, I was able to only take one of them, fix it a little and call it good (obviously crediting the author). I'd open a PR for that but, again, I'm no Kotlin expert.

Keep up the good work!