is there something similar to

Groovy on Grails

for kotiln?

Nothing that can be found. Nearest match is Spring Boot ( #spring).

I quite enjoy using Ratpack - with a bit of work, works rather well with Kotlin

Grails does a ton of stuff... Their ORM is excellent for example and they also generate basic crud web pages. It really is like Rails in the groovy world... You can probably interop with Grails as well as it does with Java...

I was a long time Grails user (still supporting old projects, using Ratpack now); Grails' ORM is pretty good (for a Hibernate based solution anyway), you can use it standalone these days - you would have to mix Groovy and Kotlin code but that's not so crazy, and separating data access code is a good idea anyway