pfSense

When you set anything other than autoselect in igc it can't actually set a fixed speed so instead it simply removes all other speeds from the available auto-select negotiation.
Thus if you connect an i225 to something that is set to autoselect it should negotiate to selected speed like:

[22.05-RELEASE][admin@4100-2.stevew.lan]/root: ifconfig igc0
igc0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
    description: LAN
    options=e120bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,WOL_MAGIC,VLAN_HWFILTER,RXCSUM_IPV6,TXCSUM_IPV6>
    ether 90:ec:77:1f:8c:3f
    inet6 fe80::92ec:77ff:fe1f:8c3f%igc0 prefixlen 64 scopeid 0x1
    inet6 fe80::1:1%igc0 prefixlen 64 scopeid 0x1
    inet 192.168.206.1 netmask 0xffffff00 broadcast 192.168.206.255
    media: Ethernet 1000baseT (1000baseT <full-duplex>)
    status: active
    nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>

But with an i226 NIC it always negotiates 2.5G whatever you have set:

[22.05-RELEASE][admin@test1.stevew.lan]/root: ifconfig igc3
igc3: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
    description: LAN4
    options=e120bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,WOL_MAGIC,VLAN_HWFILTER,RXCSUM_IPV6,TXCSUM_IPV6>
    ether 00:08:a2:12:e2:cf
    inet6 fe80::208:a2ff:fe12:e2cf%igc3 prefixlen 64 scopeid 0x4
    media: Ethernet 1000baseT (2500Base-T <full-duplex>)
    status: active
    nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>

Tested in 22.05-rel