Offline Gains are earned when you are disconnected from the game for any period of time. Only 90% of the time the player is offline for is exchanged to offline gains, with a maximum of 24 hours.
Example: If you are offline for 10 hours, you will earn a bonus as if you were online for 9 hours (90% gain), so long as your total rest time does not exceed 24 hours. This is excluding the party bonus lost when offline.
There is no offline gains popup if you have been offline for less than 30 seconds.