Try googling for "boarding ramp" instead of gangway. You might get more hits in terms of buying one.
Haven't built this specific item before, but it would certainly be easy enough. Heavy duty hinges on the dock side. Attach starboard to the bottom side of ramp, where it would contact your boat -- plus a generous margin to allow for tidal movement and the typical movement of the boat while people are boarding. Use gas springs from the underside of the ramp to the dock/pier to (1) keep the ramp in a vertical position when not in use and (2) aid in lowering/lifting ramp. Or, use a damper (I think this is the right term - basically, a "reverse" gas spring) on the topsides. If you could install a vertical post on the dock (like a 4x4) you could just figure out a little rope/pulley system for deployment/retrieval of the ramp with a jam cleat or cam cleat being used to secure the rope (since you may not be able to reach a normal cleat mounted to the dock.