I don’t think the rose bowl is gently sloped.
Because half the stadium is below ground level, they could do a lot of things to get the upper level fans close without expensive support structures or using an upper deck.
The lack of luxury boxes also keeps things compact.
The slope is gentle, obvious to the eye….and while the luxury box issue is valid, the lack of an upper deck actually means many seats are farther away from the field than many seats in the newer stadiums. The Rose Bowl stadium is a great old classic stadium, but it has been surpassed in many ways by the newer stadiums, and does not compete in the TV era for fan experience for maybe 25 thousand of the 92 thousand seats it holds.