I think you're either overrating Gordon or underrating Lee if you think Gordon can contribute what Lee can defensively. Lee gave Wade a very difficult time in the two games against Miami I've watched so far when he was assigned to him. A much harder time than most players give him.
On Eric Gordon's defense ...
Watch Eric Gordon play defense on D-Wade sometime. He will surprise you. He is very effective defensively against D-Wade.
Overall, Gordon is a good but unexceptional defender but that is too simple a summary. It doesn't describe him well enough. Eric Gordon has very specific strengths and weaknesses and can vary between extremely effective defensively to a weak defender (liability) depending on the matchup.
His limitations defensively revolve around his lack of height / length and how he can be vulnerable to bigger two guards who can shoot over the top of him (especially post up guards) ... but Eric Gordon also has absolutely top notch lateral quickness (combined with impressive physical strength) and is one of the best in the league at his position at denying dribble penetration.
Eric Gordon is extremely effective at guarding an attacker of D-Wade's ilk. A fellow undersized two guard who doesn't have the size to take advantage of Gordon's defensive limitations + a player who (Wade) relies on explosive quickness and dribble penetration to create most of his offense + is not the strongest jump-shooter. This is exactly the type of player that Eric Gordon can really get under the skin.
Now, Joe Johnson in Brooklyn. That is trickier. JJ has the size to abuse E.Gordon as much as he wants down in the post (plus can shoot over the top from distance) and there is very little Gordon can do about it. That is the only matchup in the East playoff picture that I can think of that would cause Gordon problems. Chicago (Rip Hamilton), New York (Kidd, JR Smith), Indiana (Paul George), Atlanta (Korver, Harris) and Milwaukee (Monta Ellis).