"For example, repeated copying between new collection instances can impact GC performance, and calling a method that accepts non-null types adds a method call for the null check (though you can disable runtime null checks in the compiler with -Xno-param-assertions)." so it is okay to use it on Android's app build.gradle?

Impact does not seem to be big IMO (I am using it on Android apps that are running on much slower devices than computer running Gradle build). I would not worry about performance unless it really slows down your app or decreases build time. Even if this happen you can use

Gradle profiles

to profile build times.