As
described by TripIt:
Google Calendar no longer supports floating time zones.
...
This does mean that your calendar will convert all event times to your current time zone setting. However, if you change your calendar's settings when you travel to reflect the local time zone, these items will show up in the correct time slot once you're in those locations.
I think that's an inconvenient way of managing timezones, but if you disable "Automatically adjust time zones" then TripIt sets all times as "floating", i.e. local time. But when imported to Google Calendar, it does not support floating timezones and assumes it is in the UTC (GMT) timezone and adjusts the times unnecessarily.
I've written a script that fixes the problem by letting you specify the timezone that your Google Calendar is set to.
https://calendar-177408.appspot.com/ics-timezone.php