we are closest to the chimps and bonobos, not Gorillas.
I think it is possible that closely related species crossed which would introduce genes from different lines, e.g., chimps and gorillas. Or australopithecus and chimps or ad infinitum.
There have been instances where some chimps seem to have some human genetics, sailors most likely. Seriously, there was a imported baby chimp who grew up in Texas that was probably a cross.
Polar bears and grizzlies have mated successfully in zoos with viable, fertile offspring.