i am remoldeling a cabin that has had a gas furnace in it that i have tossed. the area has a rock floor, a flue approx 3 feet from bottom of floor, and gas hookup. I want to build a rock masonary gas firplace. I have blocks, brick, stone, know how to do masonary work, gas logs, and no knowledge on how to construct the fireplace. where can i find plans? every website i have visited wants to sell me something, any help much appreciated.

the reason that you can't find anything is that you can't just build a gas fireplace. They have to be built exactly right or they can emit carbon monoxide. You will have to buy either an insert or a log set and then build your firebox and mantle around that.
Best of luck