UPDATE, 5/16, 1:45 p.m.: Via Anthony SanFilippo, the Flyers say they have not heard from Gustafsson and his agent and were not aware that he was planning on making this move, and that they still plan to extend him a qualifying offer to hang on to his rights as expected. Original post below.
Gustafsson, who turned 25 last December, played in 31 games this year for the Flyers. He put up two goals and eight assists in those games. He was one of the quicker skaters on the team's blueline (if not the outright quickest), and when he was in the lineup his speed was generally welcomed.
However, he was in and out of the lineup all season long and often found himself stuck in the press box for long stretches of time (i.e. most of January and March). And when Gus remained scratched in Game 5 of the series against the Rangers in favor of Hal Gill, of all people, it seemed fairly obvious that the Flyers don't see him as a particularly important part of this defense.
Gus wasn't much more than a good third-pairing defenseman here, but speed and inexpensiveness are all things the Flyers could probably have used on their defense. Instead, he'll head to Avangard Omsk in the KHL, where presumably he'll get more ice time.
Since Gustafsson is a restricted free agent, the Flyers can retain his NHL rights by sending him a qualifying offer this June. He (obviously) won't accept it, but it means that if he attempts to come back to the NHL in the near future, the Flyers will have the chance to play/trade him.