If gamma rays undergo pair production is there a way to say, deflect and collect the positrons using magnetic fields?
Rather than using a magnetic field, you are better off using a strong electric field to separate them - since the initial direction of the electron / positron is somewhat random, a magnetic field will deflect but not separate in a meaningful way. An electric field can pull the positrons one way, and the electrons the other way - regardless of their initial direction. Thus the positrons can always be directed towards your "collector"; of course you will need a confinement device (magnetic, presumably) which means you will ultimately need both. But if you use just a magnetic field, you will trap both electrons and positrons and they won't stay apart for long - it's not in their nature.