The quick and easy way might be to buy a six foot aluminum ladder, then screw a piece of 1/4" plywood on top of it as the footing.
Plans for a gangplank...I kind of have to cringe at the thought. I mean, it is two rails, some crossmembers, and something to walk on, and if you need plans for that, you may be best off simply buying one. Like, the ladder plus something to lay on top of it. The ladder will be about the simplest, lightest, strongest piece of fabricated aluminum you can find for the job, and the best way to do it. Bolt two "cart" wheels on the dock end if you want it to roll instead of scrape as the gangway moves. The guys in Ace Hardware or any similar shop can set you up.
Hey sorry the 'plans' comment. It was really about trying to figure out what the least of mount of structure would need to be built to hold a specified weight. You are right, the building it was easy, I just want to build it with the least amount of material.
The ladder/plywood is good.
However, I would question the ability of anyone in an Ace Hardware store to come up with anything that is practical. Ace Hardware stores are like 7-11 to grocery stores. But then again, some of the bigger home repair stores are manned with the same types of blethering idiots.