gpg --secret-keyring /mnt/usb/secring.gpg --keyring /mnt/usb/pubring.gpg -o decrypted.tgz --decrypt encrypted.tgz.asc
Why compress your files before encryption?
Answer: Compression reduces redundancy (repeated information) in your source files which could be spotted by the person attempting to break your encryption. It makes your source file look more like random data so that anyone trying to brute force crack your encryption will have a harder job identifying when they've broken it.