Hello, I'm Dr. Deb and will do my best to help you today.
Gravol or Dimenhydrinate can be given to cats although I typically suggest its use for motion sickness. It can cause sedation or lethargy, though, so it might not be a bad choice, under the circumstances.
The average 10 lb cat's dose would be 1/2 of a 50 mg tablet about 1 hour prior to the trip.
You could also consider Diphenhydramine (aka Benadryl here in the States) which is also an antihistamine. It's not commonly used as an anti-nasuea drug but it, too, can often cause sedation. The dose for a 10 lb cat would be 1/2 of a 25 mg tablet.
I usually suggest that owners perform a trial run prior to the actual trip if at all possible. It's always helpful to know what sort of side effects these drugs might cause in the patient (if any) since they can be just the opposite of what's commonly seen. For example, they might become agitated rather than sedate which is the last thing you'd want on an airplane trip!!
I hope this helps and that you all have an uneventful flight. Deb