I am using ESP8266 (2.4GHz wifi) in my product which has metallic enclosure on outside but only plastic on the other side. I will be placing this device outside home near gate. Will it be problematic for the wifi to be in metal enclosure. (Plastic side will be towards the router). Do i need to put window or will it work without window?
Generally, if your metal enclosure is grounded and thick enough, congratulations, you might have built a faraday cage and no RF enters or leaves.
In reality, the thing is probably not a perfect Faraday cage, and you're just greatly reducing the performance of your Wifi by abusing the enclosure as a very bad antenna element.
So, yes, a plastic window would sound wise.
Also note that unless you really seal the enclosure in a dry state, you might want to ensure that condensation doesn't happen on the electronics. That can be a hard problem for outdoor electronics, depending on the climate.
Read up on slot antenna's, and cut a hole in the conductive metal box of the appropriate size for the WiFi band of interest. (Hole can be weather sealed by something non-conductive and RF transparent).