Apprentice Himangshu ...Gmail scans your email content to target YOU with relevant advertising. The scan has exactly NOTHING to do with viruses....and EVERYTHING to do with invasive advertising....to the point of a technical breach of your privacy [but of course you sign that away when you join].
If a 'thing' scans your email CONTENT for keywords to determine what advertising is relevant to YOU then your information content of your email is being intercepted by a party OTHER THAN the one to which it was addressed.
Anyone with half a brain can see that as intrusive...but most are blinded by the gigs of storage...and cliquish 'invites' that make this crap advertising venture look groovy.
Perhaps you're a Gmail employee....or are getting some kick-back for making them sound good....but they ain't. It's a cheap but clever way of doing 'spam' without clogging your intray....