Medications effect different people differently. We've seen that in just this thread; responses to tramadol have ranged from drowsiness to nothing to stimulant. The same goes for gabapentin and any combinations - medication interactions.

Most meds that have the known propensity warn patients on the prescription label not to drive/operate machinery (at least at first) while taking them. After a time (also different for everyone) as the system gets used to the medication, tolerance develops. Some longterm chronic pain patients on high doses of opioids have no impairment while driving, while others taking miniscule doses for a short time may be loopy.

No hard/fast rules here.

Doc