I install the water heater, the inspector requires to verify the gas size pipe and provide the isd metric, please show me how?? I need help...

If you need to provide the ID, you will need to get to an open pipe and use a pair of calipers that can measure ID.

We don't use much gas in my area but the gas water heaters we've installed are piped with either black iron pipe or soft copper at 3/4" (19.05 mm)ID. Does that help? Cheers. Tom