I have downloaded a video attachment from my gmail account using gmail app. I am able open the video by opening the email. But where is this video saved in my phone ? Can i access this video file using Files app ?
You cannot see it in files because whatever you download within the official Gmail client will be saved in internal memory (which is completely hidden). Only .docx files (downloaded within Gmail client) can be saved to external storage using Office (using "save as") in WP.
Check your storage settings where you can change the place an download should be installed. By default the video download in the phone->videos. You cam always see it via the photos hub.