I've worked at both companies, and I think some of the similarities are due to people working one one game and then going over to the other studio to work on the other. They're both in the Dallas area. I'm sure the visuals of RAGE played some part in the art style change of Borderlands, and I've seen a lot of visual elements in RAGE that may have come from people who used to work at Gearbox. So yeah, games aren't made in a vacuum.