For CNG, you have to set plug gap to 0.6mm - 0.7mm (preferred), basically a little less compared to petrol. You will notice better performance, quieter engine as well as better fuel efficiency. CNG needs less gap to burn efficiently.
I experimented all gaps btw.
They come pre-gapped, but at 0.9-1.1mm, which is far away from cultus recommended value within manual, that's why you have to adjust new plugs gap, and also after 10k to 20k km.
21mm worked in my case. You have to put goti separately in plug area first and then attach that rod later. Doing in cold engine is recommended.
Feeler gauge that mechanics use have their readings vanished, so they are only doing it on rough estimate btw.