Purulence gives an HP bonus to units that can transmit disease.
Gift of Plague allows all of your units a disease attack.
Purulence does not give an HP bonus to units affected by Gift of Plague.
Not sure if that's working as intended. but the description says "all units that can transmit disease."
Gift of Plague definitely works. Didn't check to see if Purulence works on units that inherently have disease attacks.