There is no "according to"... that is historical fact.
I don't plan on sitting here and writing it all out, but you can find a lot of info online.
The same engineers that made the CETME moved back to Germany to join H&K and develop the G3. There are so many similarities that there is a short list of parts that don't swap and a VERY long list of ones that do.
I highly suggest jumping over and joining me on the G3/CETME forum on G&G: http://www.gunandgame.com/forums/g3-cetme/ since this has nothing to do with Open Carry in Washington State... unless you plan on OC'ing it (don't knock it, I've done it from my house to my car... )