Siempre Mo is trained by Doug O'Neill, who guided I'll Have Another to recent victories in the Kentucky Derby and Preakness.
Siempre Mo will take on five other older horses at six furlongs in the $12,500 claiming race. He will be ridden by Joy Scott. The 4-year-old gelding won his March 31 start by 2 1/4 lengths at Santa Anita.
Bryant and Gasol are part owners along with Santa Anita CEO Mark Verge, who says Gasol has been a regular visitor at O'Neill's barn since the Lakers lost in the second round of the playoffs.
O'Neill says Siempre Mo has a "big shot" to win Sunday's race. He says Gasol is in love with all the horses in the trainer's barn.
Bryant and Gasol bought into the gelding during a Lakers charity fundraiser in April. They engaged in a bidding war, and Bryant outbid Gasol by $8,000. Verge then offered Gasol a 25 percent interest if he made a separate donation to the team's foundation.