According to David, they are from an extraterrestrial planet named Orion; nothing is known about planet Orion besides they are from that planet. The Gadoids are called "Reptilians" as a recongized euphemism, even if it's a bit "redneck".
Furthermore, they came to Earth eons ago and formed the Order of the Illuminati. Than now the Gadoids want to dominate the human race and spread the entire "Fourth level". And also they've turned people into "slaves", presumabley to be the Infected.
If what David is saying is true, then the Gadoids are real and they're attempting to dominate the human race. If not, then he's just a lunatic.
The strange rocks easter egg all but confirms the aliens as being real.