post("/oauth/token") {
    val gt = call.parameters.get("grant_type") ?: call.respond(HttpStatusCode.BadRequest, mapOf("error" to "no grant_type specified"));
    call.respondText(call.request.headers["Authorization"] ?: "No header")
}

how to response and return to avoid run the second response?

Something like this?

post("/oauth/token") {
param("grant_type") {
handle {
    val gt = call.parameters.get("grant_type")!! call.respond(HttpStatusCode.BadRequest, mapOf("error" to "no grant_type specified"));
    call.respondText(call.request.headers["Authorization"] ?: "No header")
}
}
}

No, I want just want to finish the response if no grant type found.

Why dont you just do

if (gt == null) {...}

?

to avoid if