Philadelphia will retain 30 percent of Gudas' salary.
Gudas, 29, has one year remaining on his deal at a cap hit of $3.35 million. The 32-year-old Niskanen has two more seasons left on his contract at $5.75 million annually, according to CapFriendly.
Niskanen ranked second on the Capitals in average ice time (21:56) this past season, but posted his lowest point total since 2012-13 with 25 in 80 games.
Gudas managed 20 points in 77 contests while averaging just under 18 minutes.