Kyle Boller also threw a touchdown pass and Swayze Waters kicked two field goals for the Raiders (3-1), who were more focused on staying healthy than winning their final exhibition game.
Charlie Whitehurst threw a touchdown pass and former Raider Louis Rankin scored on a 99-yard kickoff return for the Seahawks (1-3), who came out with the same strategy as Oakland. Olindo Mare kicked three field goals for Seattle.
Gradkowski started in place of Jason Campbell, who was knocked out of last week's game against San Francisco because of a stinger and a sore right wrist. Campbell was on the sideline and not in uniform as Gradkowski led Oakland to its third opening-drive touchdown in three weeks.
There were plenty of other starters watching as both teams tried to stay fresh for the season opener Sept. 12. The Raiders begin the season in Tennessee, while the Seahawks will host San Francisco on opening weekend.
Quarterback Matt Hasselbeck, receivers T.J. Houshmandzadeh and Deion Branch, linebacker Lofa Tatupu and defensive backs Marcus Trufant and Lawyer Milloy were among the Seahawks to get the night off.
Running back Darren McFadden, receiver Louis Murphy, cornerback Nnamdi Asomugha, defensive linemen Richard Seymour and Tommy Kelly, and linebackers Rolando McClain and Kamerion Wimbley were among the healthy Raiders sitting out. Quarterback Jason Campbell (stinger, right wrist) and running back Michael Bush (broken left thumb) were not in uniform.
There were only a few more fans in the stands than starters on the field as most chose to stay away from the final exhibition game of the preseason. Those who did come saw another fine preseason performance from Gradkowski, who also threw two TD passes last week against San Francisco.
The Raiders scored an opening drive touchdown for the third straight exhibition game although this time it came from Gradkowski and the backups this week.
Gradkowski dumped a screen pass off to undrafted free agent Manase Tonga, who rumbled 32 yards for the score. A 34-yard pass to Darrius Heyward-Bey -- one of the few projected starters to play -- set up a 41-yard field goal by Swayze Waters on the second drive to make it 10-0.
Oakland's second touchdown was set up when Stevie Brown recovered a muff punt by Justin Forsett at the 16. Gradkowski capped it with a 9-yard TD pass to Johnnie Lee Higgins to make it 17-3.
Gradkowski finished 7 for 14 for 128 yards and the two scored before turning it over to Boller to start the second half. Boller led one touchdown drive, completing a deep pass to rookie Jacoby Ford and a 16-yard TD pass to Nick Miller.
Whitehurst got off to a slow start for the Seahawks as he tries to establish his role as a backup. But he started clicking late in the first half, throwing for 210 yards and a 26-yard touchdown pass to Deon Butler.
Whitehurst had three long completions over Chris Johnson, who did not fare well as he competes against Stanford Routt for a starting cornerback spot on Oakland. Golden Tate, Ben Obomanu and Butler had the three catches for 101 yards against Johnson.
Whitehurst could have had an even bigger day except a 45-yard TD pass to Cameron Morrah on the final play of first half was negated by a holding call on Mansfield Wrotto.