ErrorResponse is your custom response object
Kotlin
val gson = Gson()
val type = object : TypeToken<ErrorResponse>() {}.type
var errorResponse: ErrorResponse? = gson.fromJson(response.errorBody()!!.charStream(), type)
Java
Gson gson = new Gson();
Type type = new TypeToken<ErrorResponse>() {}.getType();
ErrorResponse errorResponse = gson.fromJson(response.errorBody.charStream(),type);