So if that's the case, I even not necessary to turn on QOS if I do not have IP phones in the enviroment? The GW is connected to PBX and talk to each other over the IP WAN H323.

You can do that. But if you want favourable treatment for that particular traffic when traffic goes from one port to another, queueing, congestion avoidance mechanism etc, you will need to enable QOS on the switch.