Gradkowski completed 13 of 20 passes for 141 yards. Houston reached Tampa Bay's 34-yard line on its ensuing possession, but the drive stalled when Jared Zabransky threw three straight incompletions.
Luke McCown started for Tampa Bay and completed 15 of 20 passes for 203 yards with three touchdowns.
The Buccaneers finished the exhibition season at 3-1 and will begin the regular season at Seattle.
The Texans (2-2) will open at home against Tampa Bay.
Dennis Rodman pledges to end trips to North Korea
Teacher apologizes for showing sexual image of herself in class