Combine first 6 ingredients in a small bowl. Sprinkle pork with 2 teaspoons salt, and rub pork with the garlic powder mixture.
Combine onion and remaining ingredients except cooking spray in a small saucepan over medium heat; bring to a boil. Cook 4 minutes or until guava paste dissolves, stirring constantly. Reduce heat, and simmer 4 minutes or until thickened, stirring occasionally. Remove from heat.
Place pork on grill rack coated with cooking spray; grill 20 minutes or until a thermometer registers 160° (slightly pink), turning pork occasionally. Brush guava mixture over pork. Let stand 10 minutes before slicing into 1/2-inch-thick slices.