The medicine gabapentin has a half life of 7 hours so it should start leaving your system and body within 7 hours.
Within 1 day the gabapentin should be almost completely out of your system and by 2 to 3 days the gabapentin should be completely out of your system.
Sometimes a urine test may still detect the gabapentin in your system for up to 3 days so if you need to take a urine test it's best to avoid taking gabapentin for 4 days before taking the urine test or make sure there is enough time for it to clear out of your system before the urine test.