Pega doesn't have an official suggestion for garbage collection policy. Normally, the default (gencon) works fine, and I wouldn't change it unless you run into issues specifically with garbage collection, which is rare. Gencon is good for applications with many short-lived objects, which describes Pega well.
If you have specific performance needs, balanced or optavgpause might be useful, but pretty much all you can do here is move around how often and for how long garbage collection occurs. Gencon is a good balance between all the various performance needs, which is why it's default.