1- You can create a custom command that gets the ResponseCode field and outputs text into the same or an additional field. A simple python code will help to unzip the value. This will work on search time.
2- You can use scripted input for data input which will read file etc, and send unzipped/transformed data to Splunk. This will be index time operation.
First option will be easier since Splunk will handle all file monitor processes. You will deal only with a field, otherwise you will be responsible all file operations, monitoring file EOF etc.