I believe the G5 only has the ability to do selective call with P25 systems. I have not found a way of scanning a TG and having it alert when it receives the correct tone like it does in conventional monitor mode. There are also some cases where the tone feature does not work on P25 based on some of the other forum posts on this feature.
You should be able to program the tones under P25T Sub-Group ID using the same group as your VHF tones. After you do that, you can setup a knob with Trunking TG-Scan and use the Binding Feature: Sub-Group Feature. I would suggest enabling the Delay N Function and setup a delay, especially if your dispatch does not air the information immediately after toning out. You can then scroll down to Talk Group List, add the dispatch talkgroup, and then access and enable the appropriate Sub-Group Settings right below that list.