Regarding the compression/uncompression method, it is provided OOTB by Adobe Campaign unfortunately.
So you must call an operation system script activity (command line) in your import workflow in order to do the unzip job.
If Linux server, you can use gunzip command. If Windows server, I do recommend you to install 7Zip tool (free and very powerful, and all compression algorithms supported).
Please take care that recent AC build prohibits use of system script activity in workflow, so you would have to allow the commands to allow (unzip, gpg) on the server themselves (or ask to Adobe for configuration level if they can accept it, if not hosted On Premise).