Yes, you can put a GSR crank into an LS block, but you have to use the GSR rods. There's really no point in putting a GSR crank in- the stroke geometry gain isn't worth the trouble and displacement loss. Unless you just happen to have all these parts laying around and it's the cheapest route for you, I would stick with the stock LS crank- or better yet- run the LS crank in the GSR block.