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.
Article Id 191137
This article explains how to setup SPAN (Port Mirroring) using ports associated to underlying switch chip/driver.

SPAN (Port Mirroring)

The Switch Port Analyzer (SPAN) feature is now available for hardware switch interfaces on FortiGate models with built-in hardware switches (for example, the FortiGate-100D, 140D, and 200D etc.)

To enable SPAN on a hardware switch via the GUI, go to System > Network > Interfaces and edit a hardware switch interface.

By default the system may have a hardware switch interface called LAN. A new hardware switch interface can also be created.
  • Select the SPAN check box, then select a source port from which traffic will be mirrored.
  • Select the destination port to which the mirrored traffic is sent.
  • Select to mirror traffic received, traffic sent, or both.

SPAN can also be enabled in the CLI:
config system virtual-switch
edit <port>
set span enable
set span-source-port <port>
set span-dest-port <port>
set span-direction {both | Tx | Rx}