Make sure the VLAN you want to have untagged on the port (in this example VLAN 203) is part of vlan-allowed and vlan-untagged. Then you set the actual untagged VLAN via pvid.
interface GigaEthernet0/2 switchport trunk vlan-allowed 201,203,205 switchport trunk vlan-untagged 203 switchport mode trunk switchport pvid 203