Hi Tae,
You will not able to able send the log file as a compressed file from firewall.The logs file can be saved to TFTP server in .txt fomat. For Example, You can try to save the output of the get commands following the below command:
Eg. get session > tftp <IP-Address> <Filename>
Eg. get db str > tftp <IP-Address> <Filename>
Regards,
Rishi Surana