As I recallin the movie, Forrest Gump and Jenny are finally reunited at the home where Forrest grew up in.
They were relaxing in the House when Forrest began to questionJenny’s love for him. He then remarked sarcastically about his disability and love for her saying... “I may not be a smart man, but I know what Love is!”
I don’t recall Forrest saying anything about being “smart enough” in fact if anything, Forrest would always be questioned by people if he was stupid or something.