There is a problem with items "FortiGate: Used memory", "FortiGate: Memory utilization", "FortiGate: Total memory" and "FortiGate: Available memory" in both 6.0 and 6.4 official zabbix template "FortiGate by SNMP"
In the template "Fortigate: Used memory" use OID 1.3.6.1.4.1.12356.101.4.1.4.0 and unit is in B whereas this OID is the current memory utilization in percentage. So this OID should be used straight for the item "FortiGate: Memory utilization"
In the template "Fortigate: Total memory" is in B whereas the OID is in KB so this item have to be multiply by 1000.

Please find attached, the firewall memory utilization via get system performance status and official fortigate value and the modified template with the good values.

As you can see on the screenshot all memory items in the official template are not correct.

How it is in the template :

        - uuid: 5ccef71de6944c91bf7d1e330782b542
          name: 'FortiGate: Total memory'
          type: SNMP_AGENT
          snmp_oid: 1.3.6.1.4.1.12356.101.4.1.5.0
          key: 'vm.memory.total[fgSysMemCapacity.0]'
          history: 7d
          units: B
          description: |
            MIB: FORTINET-FORTIGATE-MIB
            The total memory allocated for the tasks.
          tags:
            - tag: component
              value: memory
        - uuid: 15fb0d4185f04b4fb8e2efc6d0ef514c
          name: 'FortiGate: Used memory'
          type: SNMP_AGENT
          snmp_oid: 1.3.6.1.4.1.12356.101.4.1.4.0
          key: 'vm.memory.used[fgSysMemUsage.0]'
          history: 7d
          units: B
          description: |
            MIB: FORTINET-FORTIGATE-MIB
            Current memory utilization (percentage).
          tags:
            - tag: component
              value: memory
        - uuid: 29a7ec8007ef44b0b473f1c16be11059
          name: 'FortiGate: Memory utilization'
          type: CALCULATED
          key: 'vm.memory.util[memoryUsedPercentage.0]'
          history: 7d
          value_type: FLOAT
          units: '%'
          params: 'last(//vm.memory.used[fgSysMemUsage.0])/last(//vm.memory.total[fgSysMemCapacity.0])*100'
          description: 'Memory utilization in %.'
          tags:
            - tag: component
              value: memory

How it should be :

        - uuid: 9822c2939b984fd4bef5fac7f5c73c61
          name: 'FortiGate: Total memory'
          type: SNMP_AGENT
          snmp_oid: 1.3.6.1.4.1.12356.101.4.1.5.0
          key: 'vm.memory.total[fgSysMemCapacity.0]'
          history: 7d
          units: B
          description: |
            MIB: FORTINET-FORTIGATE-MIB
            Total physical memory (RAM) installed (KB).
          preprocessing:
            - type: MULTIPLIER
              parameters:
                - '1000'
          tags:
            - tag: component
              value: memory
        - uuid: 30f9d69d69bb45a5833aa27f71eb5240
          name: 'FortiGate: Memory utilization'
          type: SNMP_AGENT
          snmp_oid: 1.3.6.1.4.1.12356.101.4.1.4.0
          key: 'vm.memory.util[fgSysMemUsage.0]'
          history: 7d
          value_type: FLOAT
          units: '%'
          description: |
            MIB: FORTINET-FORTIGATE-MIB
            Current memory utilization (percentage).
          tags:
            - tag: component
              value: memory