vlan as gateway

> 1) when we define self ip on ltm1/2 as 192.168.100.4 & 192.168.100.5 , what should be the traffic group ?

Non-floating self-IPs are always assigned to traffic-group-local-only

> 2) how to define the floating traffic group on ltm which will server as gateway (192.168.100.6) for the servers in this subnet?

You do not need a new floating traffic group for your new vlan - just use traffic-group-1

As you have an Active-Standby HA group, you want all your floating self-IPs to move if the active device fails, so you only need one floating traffic group. If you wanted an Active-Active or Active-Active-Standby HA group, you need multiple floating traffic groups to distribute some of the traffic to the Active devices (at a minimum, one traffic group per active device).