FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article describes when No new firmware is listed in the Firmware Management section from GUI.

1) Run the below commands to make sure that the FortiGuard Distribution Network has the images updated on FDN Servers.
# diag fdsm image-upgrade-matrix
# diag fdsm image-list
# diag fdsm fds-update
The above commands will list all the compatible firmware images & versions which are retrieved from FDN, if this is successful then there are no issues communicating with the FortiGuard distribution list.

2) Enable the below command:
# config system central-management
    set allow-remote-firmware-upgrade enable
This will enable remotely upgrading the firmware on this FortiGate from the central management server (FortiGuard by default).

After this change, it will be possible to find the recommended firmware: