Garmin G300 Pilot's Manual page 214

integrated autopilot
At 50 feet from the VNAV Target Altitude, the Integrated Autopilot
automatically transitions from VNAV Target Altitude Capture to Altitude
Hold Mode and selects the VNAV target altitude as the new Integrated
Autopilot Altitude Reference. As Altitude Hold Mode becomes active,
the white 'ALT' annunciation moves to the active vertical mode field and
flashes green for 10 seconds to indicate the automatic transition.
Selected
Altitude Below
Automatic Transition to ALTV
VNAV Altitude
Mode with ALT Mode Armed
Automatic Transition
from ALTV to ALT Mode
Altitude
Reference
Automatic Mode Transitions During Altitude Capture
196
Garmin G300 Pilot's Guide 190-00921-00 Rev. D