pfSense

Default GRE MTU value is 1500 (it should be 1476 as maximum for ethernet),
This is impossible in most cases when GRE/IPsec is used

It can be useful to set default MTU value for such cases to 1400
by using such logic:

if ((GRE SOURCE == IPSEC PH1 SOURCE) && (GRE DESTINATION == IPSEC PH2 DESTINATION) &&
    (IPSEC MODE == TRANSPORT) && (IPSEC == ENABLED) && (GRE MTU is not manually entered)) {
            $GRE_MTU = 1400;
}