I read it to mean that he accessed his GMail account while using PureVPN, which really raises more questions than it answers because PureVPN should have no way to know which GMail account is being accessed, regardless of what logs they keep.
Google discloses that the victims account was accessed by a set of IPs. Those IPs are known to belong to PureVPN. A list of people connecting to those IPs as clients is requested by the FBI. The connection logs validate their suspicions.
Also login time, connecting IP and logout time. So no traffic is stored, just that you logged in and were connected for X amount of time.