I wrote to denso on this issue and here was their reply:
Dear Sir,
We think that IK20(Gap:1.1mm) is suitable for CNG.
However,CNG engine is recommended to use spark plug with small spark gap(0.8mm).
to lower demanded voltage. In addition CNG engine seldom occure carbon fouling of the
insulator.
Therefore, the spark plug with a higher heat range(#22) can be used.
So we recommend the following spark plug to your Honda city with CNG .
VK22 ( DENSO p/n : 267700-0730, so called Iridium tough )
*The gap of VK22 is 0.8mm.
We look forward to your next visit to the DENSO Website.
Best regards,
DENSO CORPORATION
1-1 Showa-cho, Kariya, Aichi 448-8661, JAPAN
URL: http://www.globaldenso.com/