Just out of curiosity...what is the difference between a gmc 5.7 L vortec and a chevy 5.7 L vortec engine?

GM engines are used interchangeably across the product line. I remember several years back when some consumer was upset because his brand new Cadillac had the same engine as a Chevrolet. Back when I had my 78 Trans Am it had a 400 cid Pontiac engine in it which was blue, and the 79 had a 403 cid Chevrolet engine in it that was painted orange, but I think they have done away with situations like that. With possible exception of some hang on components the two engines you reference are likely identical.

Just nitpicking here but the 403 was on OLDS engine.

Right you are. As soon as I read your post I recalled it being Olds, not Chevy. It was a dog compared to the Pontiac 400. Mind you, the Pontiac 400 was no great shakes ... rated at 220 HP in the high performance version. The options were a TA6.6 and a 6.6. Fun to drive though, especially with the 4-spd manual transmission.