Gregerson, obtained last month from the St. Louis Cardinals as the player to be named in the Khalil Greene deal, didn't allow a run in five spring appearances, allowing two hits in six innings with eight strikeouts.
Lopez, who last week looked like he had locked down a spot in the bullpen, posted a 2.13 ERA in 11 spring appearances with 13 strikeouts in 12 2/3 innings.
The Padres also announced that right-handed pitcher Cha Seung Baek has been placed on the 15-day disabled list -- retroactive to March 30 -- with a strained right forearm.
Also, pitcher Wilton Lopez was designated for assignment and the contracts of pitchers Kevin Correia, Shawn Hill, Edwin Moreno, Duaner Sanchez, Walter Silva and Gregerson were purchased.
Arturo Lopez was reassigned to Minor League camp and pitchers Mike Adams and Mark Worrell were transferred from the 15-day disabled list to the 60-day disabled list.
The Padres open the regular season at 4:05 p.m. PT on Monday at PETCO Park against the Los Angeles Dodgers with nine players making their first Opening Day roster.
Also Sunday, Hill, who will be the fifth starter in the rotation and pitch Friday against the San Francisco Giants, tossed five scoreless innings against Minor League hitters during an intrasquad game in Arizona, striking out four and allowing two hits.
The Padres, 25-man Opening Day roster:
Catchers: Nick Hundley, Henry Blanco.
Infielders: Adrian Gonzalez, Edgar Gonzalez, Everth Cabrera, David Eckstein, Kevin Kouzmanoff, Luis Rodriguez.
Outfielders: Scott Hairston, Chase Headley, Brian Giles, Jody Gerut, Cliff Floyd.
Pitchers: Hill, Gregerson, Moreno, Ed Mujica, Jake Peavy, Chris Young, Correia, Silva, Sanchez, Heath Bell, Cla Meredith, Eulogio De La Cruz.
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.