This is correct but it should be noted that if one owns these games on Steam and clicks on one of them in the library, there is a "CDKEY" option on the menu with which one can obtain a CDKEY from their Steam game (which is NOT the Steam key as you say), and that CDKEY can be redeemed on GOG. Allegedly however, not all copies of the game on Steam have this CDKEY however, it may depend on where it was obtained or other factors. I own Clear Sky on Steam which I won in a giveaway somewhere IIRC, and that copy did redeem on GOG no problem when I used the CDKEY as mentioned above.

So oddly enough, CDKEYs may exist for these games even if one does not own them on CD/DVD.
That is up to the developers/publishers of the game, not GOG. You'd have to talk to the publishers directly, and some of them do in fact offer Steam keys for their games upon proof of purchase. They may require a screenshot of a digital receipt, a copy of an email receipt, or photocopy of a paper receipt or similar in order to qualify however, and they may not accept gift-copies where no receipt exists as proof of purchase.

GOG really isn't part of that equation, even if someone would like them to be. I'd like Microsoft to give me free copies of Flight Simulator X Deluxe on Steam, as I own it on DVD here. Not likely going to happen. I'd like Walmart to give me a free copy of the toaster oven I bought at Canadian Tire that's wearing out. I'm doubtful but hopeful.