Colonoscopies aren't quite general anesthesia in the US (generally, I'm sure there are exceptions). General anesthesia as being completely unresponsive to external stimuli and potentially needing supportive measures to make sure you maintain your airway.

What they tend to do is "conscious sedation". Basically load you up with a sedative and painkiller - midazolam and fentanyl as an example. You're not completely out of it, you can breath on your own and will show some response to stimuli. But you won't remember any of it due to the amnesia from the sedative.