The Google Voice account login is typically triggered by the availability or inavailability of a SIP route. In other words, if you have no devices registered and no SIP URI defined, an incoming call can't be routed through the Gateway. Thus, during this time, your account will be logged out. When you register a device or define a SIP URI, then a path becomes available for a Google Voice call and your account is logged in.
On https://simonics.com/gw/, you may force a login or logout of your Google Voice account. This is an advanced feature and it is normally unnecessary to manually change the logged-in state of your Google Voice account.