I am using experimental multiplatform features from Kotlin 1.2.60 and the build is successful only by adding @ExperimentalMultiplatform annotation to every class or function that uses it. Is there a way around this?

Yep, see here https://github.com/Kotlin/KEEP/blob/master/proposals/experimental.md#propagation-and-opt-in-for-whole-modules

Cool! Also, how do I add a compiler flag in gradle to common code (all examples are for JVM) like -Xuse-Experimental.

@Mohit Gurumukhani something like that:

compileTestKotlin {
    kotlinOptions.freeCompilerArgs = ["-Xuse-experimental"]
}