Use a blender or mini food processor to combine the garlic, grapefruit juice, lime zest and juice, 2 tablespoons oil, brown sugar, oregano, cumin and salt. Process till blended. Transfer to a large bowl and add the pork and bay leaf; toss to combine. Marinate, covered, at room temperature for 1 hour or refrigerate for up to 6 hours.
Using the "Sauté" function set on high if available, heat remaining 1 tablespoon oil in the pressure cooker (or use a large skillet). Remove the pork from the marinade (reserving the marinade), and shake the meat to remove any excess liquid. Cook until it is browned on all sides, about 12 minutes (you will need to do this in batches, transferring browned pork to a plate as you go).
When all the pork is browned, return the pieces to the pot, along with any juices from the plate. Add the reserved marinade to the pot. Cover and cook on high pressure for 80 minutes. Let the pressure release naturally.
Remove the pork from the cooking liquid. Taste the liquid and, if it seems bland or too thin, boil it down either in the pressure cooker on the Sauté setting or in a separate pot on the stove until it thickens slightly and intensifies in flavor, 7 to 15 minutes. Remove the bay leaf and adjust the seasonings.