If a person wants to keep their explosiveness during a mass gain cycle could they do some plyometrics for GPP?
You could do some plyometrics one day per week which should be enough for maintenance of explosiveness but I wouldnt call this GPP as GPP hits “general” fitness qualities. Plyometrics to enhance explosiveness are more a specific form of preparation. If you do plyometrics make sure they are of the explosive variety as doing long sets of endless plyometric drills works more on endurance. Also you could just add in an olympic lifting movement or jump squats at the beginning of a few workouts which will go a long way towards keeping your explosiveness.