This episode is based on "The Amazing Spider-Man" (Vol. 1) #121-122, "The Night Gwen Stacy Died" & "The Goblin's Last Stand!". It's regarded as one of the all-time best and important Spider-Man stories.
The final battle between Spider-Man and the Green Goblin clearly does take place on the George Washington Bridge. In the comics the fight between the two in The Amazing Spider-Man" (Vol. 1) #121-122 is said to take place on the George Washington Bridge, but the artwork clearly is drawn after the Brooklyn Bridge.
This episode's title "Turning Point!" is taken from the cover "The Amazing Spider-Man" (Vol. 1) #121.
In addition to issues #121-122, a lot of the material for this episode is taken from "The Amazing Spider-Man" (Vol. 1) #39: Where Green Goblin spies on Spider-Man changing to Peter Parker and later attack Peter Parker without his web shooters and tying him up to his glider.