Gudas signed a multiyear contract to remain with the Flyers on June 23, 2016. Washington said Philadelphia will retain 30 percent of his salary and that the contract will expire after next season.
Niskanen has two years remaining at $5.75 million per season on a seven-year contract he signed July 1, 2014 to join Washington as a free agent.
Gudas had 20 points (four goals, 16 assists) and was plus-6 this season. He has 105 points (24 goals, 81 assists) and 579 penalty minutes in 416 NHL games with the Flyers and Tampa Bay Lightning.
"Obviously you guys know that I'm playing physical hockey and I'm a simple player," Gudas said. "The first pass to the first open man, pushing away from your own net, good PK. Those are the things that I'm focused on throughout the season.
"Obviously I want to help the offense as well, but the main thing I can help -- work well with the team, work my back off and just do anything that they need me to do. I'm not picky to play a role or not. I'm just accepting what it is when it's a well-oiled machine, so that's what I'm looking forward to do."