Swayze Waters kicked a 29-yard field goal with 12:23 to play, giving the Raiders a 27-16 lead. Louis Rankin then ran the ensuing kickoff back 99 yards for a touchdown and the Seahawks were good on a 2-point conversion to draw within three.
Seattle made a goal-line stand inside the 3-minute mark and they quickly moved near midfield. Four straight incompletions, however, gave Oakland the ball and the Raiders ran out the clock.
Gradkowski completed 7-of-14 passes for 128 yards. He hit Manase Tonga with a 32-yard pass for the first score of the game and also found Johnnie Lee Higgins with a 9-yarder late in the first period.
Kyle Boller added a 16-yard touchdown throw for the Raiders in the third quarter.
Oakland (3-1) will start its season at Tennessee and Seattle (1-3) will open at home against San Francisco.